Description of problem:
When compiling for RISC-V, clang unconditionally invokes /usr/bin/ld, i.e. even when -fuse-ld=lld or -fuse-ld=/usr/bin/ld.lld is specified.

Version-Release number of selected component (if applicable):
clang-9.0.0-1.fc31.x86_64

How reproducible:
always

Steps to Reproduce:
1. echo 'int _start() {while(1);}' > foo.c
2. clang --target=riscv64 -fuse-ld=lld -nostdlib -v foo.c -o foo
3. echo $?

Actual results:
[..]
clang-9: warning: argument unused during compilation: '-fuse-ld=lld' [-Wunused-command-line-argument]
[..]
/usr/bin/ld: /tmp/foo-8bcfba.o: Relocations in generic ELF (EM: 243)
/usr/bin/ld: /tmp/foo-8bcfba.o: error adding symbols: file in wrong format
clang-9: error: ld command failed with exit code 1 (use -v to see invocation)
1


Expected results:
[..]
[.. no such warning ..]
[..]
"/usr/bin/ld.lld" [..]
0


Additional info:
It works as expected when targeting x86-64:

$ clang  -fuse-ld=lld -nostdlib -v foo.c -o foo
[..]
 "/usr/bin/ld.lld" [..]
$ echo $?
0

Workaround:

Link directly, e.g.:

$ clang --target=riscv64 -c foo.c -o foo.o
$ ld.lld foo.o -o foo
$ echo $?
0
 file foo foo.o
foo:   ELF 64-bit LSB executable, UCB RISC-V, version 1 (SYSV), statically linked, not stripped
foo.o: ELF 64-bit LSB relocatable, UCB RISC-V, version 1 (SYSV), not stripped