Add a hack to workaround ZSH as BUILDENV_SHELL breaking CPUTYPE.

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
Sponsored by:	EMC / Isilon Storage Division