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

Makefile.inc1

@@ -784,6 +784,9 @@ buildenvvars: .PHONY
 BUILDENV_DIR?=	${.CURDIR}
 buildenv: .PHONY
 	@echo Entering world for ${TARGET_ARCH}:${TARGET}
+.if ${BUILDENV_SHELL:M*zsh*}
+	@echo For ZSH you must run: export CPUTYPE=${TARGET_CPUTYPE}
+.endif
 	@cd ${BUILDENV_DIR} && env ${WMAKEENV} BUILDENV=1 ${BUILDENV_SHELL} \
 	    || true