Rewrite sw_vers makefile check to avoid error message on Linux.

When building compiler-rt on Linux, the "which sw_vers" check fails and
writes an error message into the build log. Apparently on Solaris "which"
writes the error message to stdout, so that the current test won't even
work properly. As far as I know sw_vers always lives in /usr/bin, so just
check for it there instead of using "which".