configure: Add -Werror to avx2, avx512 tests

configure: Add -Werror to avx2, avx512 tests

When using clang, we get

ERROR: configure test passed without -Werror but failed with -Werror.
       This is probably a bug in the configure script. The failing command
       will be at the bottom of config.log.
       You can run configure with --disable-werror to bypass this check.

What we really want from these two tests is whether the
entire code sequence is supported, including pragmas.
Adding -Werror makes the test properly fail for clang.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210719200112.295316-1-richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>