MIPS: Support custom compilation using GN

There is a need to compile V8 using CLANG and gcc-toolchain
option, in order to support CLANG on those targets which
don't have Debian RootFs (Release 6 and MIPS64BE).

This patch enables custom compilation using GN by
removing those flags that are set by the custom compiler.

Bug: 
Change-Id: Ia9b2f0fac1f76a2ad98d190fd307aa60229e8eeb
Reviewed-on: https://chromium-review.googlesource.com/763449
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516325}
1 file changed