toolchains: mips: prevent failing build on MacOS
mips gcc will fail to build using the system's
BSD sed. Make sure the Homebrew-installed GNU sed
'sed' simlink is in the PATH. Otherwise, only 'gsed'
can be used to run GNU sed by default.
Change-Id: I0a793c692d9a6de4f326e447281892eb08e4f4cb
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