external/boringssl: update #define guards for x86_64-gcc.c.

OS X builds with NO_ASM and was getting both generic.c and x86_64-gcc.c.
This change updates the latter so that it's excluded in NO_ASM builds.

(cherry picked from commit 53b609c9db503488fc065728d214c9db67ad5740)

Change-Id: Idf801013db636bd4d8f6c4588102c241fdb9fbf6
1 file changed