Fix several problems when building Breakpad with new NDK unified
headers.
- Unified headers define its own tgkill wrapper, so rename our own
wrapper to __tgkill.
- Unified headers define user_fpxregs_struct for all API levels, so
don't redefine it.
- Only the target sources under google-breakpad/src/common/linux should
use custom Android headers, so change the includes line in moz.build to
use OS_INCLUDES.
MozReview-Commit-ID: HGnUMu5vDUM
6706a2d1fe