*** Bug 1545168 has been marked as a duplicate of this bug. ***

There are two cases of BuildRequires with isa in gdb.spec:

(1)
# https://fedoraproject.org/wiki/Packaging:Guidelines#BuildRequires_and_.25.7B_isa.7D
%if 0%{?_with_buildisa:1} || 0%{?_with_testsuite:1}
%global buildisa %{?_isa}
%else
%global buildisa %{nil}
%endif
BuildRequires: rpm-libs%{buildisa}
...
 - Those need to be enabled by --with buildisa.
# --with buildisa: Use %%{?_isa} for BuildRequires

(2)
%if 0%{?_with_testsuite:1}
BuildRequires: gcc-gdb-plugin%{?_isa}
BuildRequires: zlib-devel%{bits_local} zlib-devel%{bits_other}
...
%endif
 - Those apply only for local rpmbuild rebuild --with testsuite.
# --with testsuite: Run the testsuite (biarch if possible).  Default is without.

So with no extra --with options gdb.spec is compliant with:
  https://fedoraproject.org/wiki/Packaging:Guidelines#BuildRequires_and_.25.7B_isa.7D