[CMAKE] Find ld64 using xcrun

Given similar reasons from r276710, ld64 scrubs DYLD_* environment if
called from the shim executable /usr/bin/ld.

Add support for finding ld64 via xcrun.

This is needed in order to get LIT to have the full path to the ld4
executable.

Differential Revision: https://reviews.llvm.org/D22791

rdar://problem/24300926

llvm-svn: 276781