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

--HG--
extra : rebase_source : f5c29c9949a48a8376f84dcc676e5f8df886f130