external/boringssl: try to fix aarch64+Clang.

It appears that the version of Clang in Android doesn't support the
.arch_extension directive. This change removes the .arch and
.arch_extension lines (because they are triggering errors) and adds a
-march option on the command line instead.

The aarch64+Clang build is still broken with this change, but it's
broken in binder rather than BoringSSL with it.

Change-Id: I32c557bdfde4df66d26794ccdd650356f2bbaf8f
5 files changed