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This CL modifies gcc_version.gni to set the gcc_version to the target
GCC toolchain version when building for MIPSEL Linux.

BUG=
TEST=Compile 'chrome' target with 'os="linux" cpu_arch="mipsel"'.
     Manually verify that the correct GCC options are used.

Review URL: https://codereview.chromium.org/797593002

Cr-Commit-Position: refs/heads/master@{#308334}