Due to issues with CMake enable_language, we have to delay setting
CMAKE_ASM_FLAGS until after enable_language(ASM) has been called.

We also need to remove the '.file' macro from x86gas.pl to prevent the
filenames from being overridden from those provided by the build
system.

Change-Id: I436f57ec45e4751714af49e1211a0d7810e4e56a
Reviewed-on: https://boringssl-review.googlesource.com/16127
Reviewed-by: Steven Valdez <svaldez@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>