Updated toolchain.gypi to support v8 using CLANG on MIPS. These changes
include using integrated assembler with CLANG, and disabling options
used by GCC which are not supported by CLANG.

Reland https://codereview.chromium.org/1519493002 with fix to remove
duplicated 'conditions' sections.

TEST=
BUG=

Review URL: https://codereview.chromium.org/1530153003

Cr-Commit-Position: refs/heads/master@{#32963}