[MIPS] Avoid using LA macro in inline assembly

LLVM's integrated assembler currently does not expand LA macro correctly
for PIC mode.
This change is replacing LA macro with appropriate instructions.

BUG=130022
TEST=Build Chromium with Clang and integrated as
TEST=Run sandbox_linux_unittests and components_browsertests

Review-Url: https://codereview.chromium.org/1908253003
Cr-Commit-Position: refs/heads/master@{#391527}
1 file changed