Add a hack to workaround ZSH as BUILDENV_SHELL breaking CPUTYPE.
ZSH considers CPUTYPE a magic variable that will be the output of 'uname -m'
even if already set in environment when starting up.
The CPUTYPE?= check in Makefile.inc1 and supporting overriding CPUTYPE
manually in the buildenv shell make automatic workarounds too tricky
here. ZSH should really respect variables set in the environment before
trashing them.
X-MFC-With: r290423
MFC after: 2 weeks
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