Fix gcc_version.gni to determine target GCC version for MIPSEL Linux.

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}

build/config/gcc/gcc_version.gni

@@ -9,8 +9,8 @@ if (is_android) {
   } else {
     gcc_version = 49
   }
-} else if (current_toolchain == "//build/toolchain/cros:target") {
-  # TODO(benchan): Generalize the check for platforms other than Chrome OS.
+} else if (current_toolchain == "//build/toolchain/cros:target" ||
+           current_toolchain == "//build/toolchain/linux:mipsel") {
   gcc_version = exec_script("../../compiler_version.py",
                             [
                               "target",