Paritally revert standalone.gypi changes in a451bd1a and introduce a new separate variable for the mkpeephole.

On big-endian MIPS, qemu is used to build the snapshot,
because there's no simulator support for big-endian MIPS.

BUG=

Review-Url: https://codereview.chromium.org/2172653002
Cr-Commit-Position: refs/heads/master@{#38103}