Use standard location for default mips linux sysroot
Use standard location for default mips linux sysroot

The mips sysroot is automatically installed in this
location by gclient hooks (install-debian.wheezy.sysroot.py).

Expect mipsel-linux-gcc to be in the PATH rather than
finding it relative to the sysroot.  This matches the
behaviour the arm linux cross compile.

Depends on this native_client change: https://codereview.chromium.org/1212573002

BUG=504471
TEST=build all with target_arch=mips

Review URL: https://codereview.chromium.org/1207323003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339945}

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