macOS: Revert "Resolve compiler in /usr/bin to that reported by Xcode xcrun"

Revert commit 1f085e11e4 (OS X: Resolve compiler in /usr/bin to that
reported by Xcode xcrun, 2015-01-03, v3.2.0-rc1~126^2~1).  The compilers
in Xcode cannot be used by build systems that do not add an `-isysroot`
flag.

Issue: #19180