sys-libs/glibc: mangle bad -march= only for ABI=x86

sys-libs/glibc: mangle bad -march= only for ABI=x86

Normally setup_target_flags() attempts to guard against
too conservative (or missing) -march= value by chacking
if sync primitives can be compiled. If it can't be compiled
glibc assumes -march= value to be too low.

Unfortunately, test always runs against 32-bit ABI without
checking if current ABI is actually 32-bit. This causes
the test mis-fire on USE=-multilib ARCH=amd64 systems.

Sweep workaround under ABI=x86 case.

Reported-by: Klaus Kusche
Closes: https://bugs.gentoo.org/700232
Package-Manager: Portage-2.3.79, Repoman-2.3.18
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>