Don't use x86_64-gcc.c with NO_ASM.

Android (on 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.

Change-Id: I1f0e1c5e551eed9c575ce632ec3016fce7ec9d2e
Reviewed-on: https://boringssl-review.googlesource.com/4741
Reviewed-by: Adam Langley <agl@google.com>