Avoid emitting popcnt in libclang_rt.fuzzer*.a if unsupported

Since popcnt is only supported by CPUTYPE=nehalem and later, ensure that
this instruction is only emitted when appropriate. Otherwise, programs
using the library can abort with SIGILL.

See also: https://github.com/llvm/llvm-project/issues/52893

PR: 258156
Reported by: Eric Rucker <bhtooefr@bhtooefr.org>
MFC after: 3 days

(cherry picked from commit 133180557479cd9676758e6f3f93a9d3e1c6b532)