I think this is upstream PR67143, except that the patch as is doesn't really work, because as can be seen in tmp-mddump.md, unlike GCC 5, GCC 4.8 didn't support using mode attributes in code attributes as the patch uses.
The "IJ" from the code attribute works fine, but instead of having there "K" or "L" (the former for QI/HI/SImode and latter for DImode for xor/ior/and, tmp-mddump.md contains unexpanded <lconst_atomic>.
The following seems to work for the above preprocessed source instead (or we could split the 3 define_insn_and_split patterns, having one for plus/minus and noather one for xor/and/ior, or
having one for QI/HI/SImode and another one for DImode.

--- gcc/config/aarch64/atomics.md.jj	2015-06-18 16:33:14.157720233 +0200
+++ gcc/config/aarch64/atomics.md	2020-01-07 16:52:29.012105743 +0100
@@ -115,16 +115,20 @@
   }
 )
 
+(define_code_attr const_atomic
+  [(plus "no,yes") (minus "no,yes")
+   (xor "yes,no") (ior "yes,no") (and "yes,no")])
+
 (define_insn_and_split "atomic_<atomic_optab><mode>"
-  [(set (match_operand:ALLI 0 "aarch64_sync_memory_operand" "+Q")
+  [(set (match_operand:ALLI 0 "aarch64_sync_memory_operand" "+Q,Q")
     (unspec_volatile:ALLI
       [(atomic_op:ALLI (match_dup 0)
-	(match_operand:ALLI 1 "<atomic_op_operand>" "r<lconst_atomic>"))
+	(match_operand:ALLI 1 "<atomic_op_operand>" "r<lconst_atomic>,rIJ"))
        (match_operand:SI 2 "const_int_operand")]		;; model
       UNSPECV_ATOMIC_OP))
        (clobber (reg:CC CC_REGNUM))
-   (clobber (match_scratch:ALLI 3 "=&r"))
-   (clobber (match_scratch:SI 4 "=&r"))]
+   (clobber (match_scratch:ALLI 3 "=&r,&r"))
+   (clobber (match_scratch:SI 4 "=&r,&r"))]
   ""
   "#"
   "&& reload_completed"
@@ -134,6 +138,7 @@
 			    operands[1], operands[2], operands[4]);
     DONE;
   }
+  [(set_attr "enabled" "<const_atomic>")]
 )
 
 (define_insn_and_split "atomic_nand<mode>"
@@ -159,17 +164,17 @@
 )
 
 (define_insn_and_split "atomic_fetch_<atomic_optab><mode>"
-  [(set (match_operand:ALLI 0 "register_operand" "=&r")
-    (match_operand:ALLI 1 "aarch64_sync_memory_operand" "+Q"))
+  [(set (match_operand:ALLI 0 "register_operand" "=&r,&r")
+    (match_operand:ALLI 1 "aarch64_sync_memory_operand" "+Q,Q"))
    (set (match_dup 1)
     (unspec_volatile:ALLI
       [(atomic_op:ALLI (match_dup 1)
-	(match_operand:ALLI 2 "<atomic_op_operand>" "r<lconst_atomic>"))
+	(match_operand:ALLI 2 "<atomic_op_operand>" "r<lconst_atomic>,rIJ"))
        (match_operand:SI 3 "const_int_operand")]		;; model
       UNSPECV_ATOMIC_OP))
    (clobber (reg:CC CC_REGNUM))
-   (clobber (match_scratch:ALLI 4 "=&r"))
-   (clobber (match_scratch:SI 5 "=&r"))]
+   (clobber (match_scratch:ALLI 4 "=&r,&r"))
+   (clobber (match_scratch:SI 5 "=&r,&r"))]
   ""
   "#"
   "&& reload_completed"
@@ -179,6 +184,7 @@
 			     operands[2], operands[3], operands[5]);
     DONE;
   }
+  [(set_attr "enabled" "<const_atomic>")]
 )
 
 (define_insn_and_split "atomic_fetch_nand<mode>"
@@ -206,17 +212,17 @@
 )
 
 (define_insn_and_split "atomic_<atomic_optab>_fetch<mode>"
-  [(set (match_operand:ALLI 0 "register_operand" "=&r")
+  [(set (match_operand:ALLI 0 "register_operand" "=&r,&r")
     (atomic_op:ALLI
-      (match_operand:ALLI 1 "aarch64_sync_memory_operand" "+Q")
-      (match_operand:ALLI 2 "<atomic_op_operand>" "r<lconst_atomic>")))
+      (match_operand:ALLI 1 "aarch64_sync_memory_operand" "+Q,Q")
+      (match_operand:ALLI 2 "<atomic_op_operand>" "r<lconst_atomic>,rIJ")))
    (set (match_dup 1)
     (unspec_volatile:ALLI
       [(match_dup 1) (match_dup 2)
        (match_operand:SI 3 "const_int_operand")]		;; model
       UNSPECV_ATOMIC_OP))
     (clobber (reg:CC CC_REGNUM))
-   (clobber (match_scratch:SI 4 "=&r"))]
+   (clobber (match_scratch:SI 4 "=&r,&r"))]
   ""
   "#"
   "&& reload_completed"
@@ -226,6 +232,7 @@
 			     operands[2], operands[3], operands[4]);
     DONE;
   }
+  [(set_attr "enabled" "<const_atomic>")]
 )
 
 (define_insn_and_split "atomic_nand_fetch<mode>"

Or we could backport whatever gen* change made the backport to work, but guess that one would require inspection of the insn* files before/after on all architectures and verify without the backport nothing changed and with the backport only the 6 expanded define_insn_and_split patterns.

Let's go with Jakub's Comment 2 patch for now.  So dev_ack+ and adding onto the wishlist.

Given this is an aarch64 problem only, fixing it in RHEL 7.9 would not help; the only way would be to issue a z-stream.  But that is reserved for critical issues only, and this BZ doesn't seem to meet the bar.  Thus closing.