x86, build: Don't get confused by local symbols

arch/x86/crypto/sha1_avx2_x86_64_asm.S introduced _end as a local
symbol, which broke the build under certain circumstances.  Although
the wisdom of _end as a local symbol can definitely be questioned, the
build should not break for that reason.

Thus, filter the output of nm to only get global symbols of
appropriate type.

Reported-by: Andy Lutomirski <luto@amacapital.net>
Cc: Chandramouli Narayanan <mouli@linux.intel.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-uxm3j3w3odglcwhafwq5tjqu@git.kernel.org