This updates the Bazel configuration for
https://github.com/llvm/llvm-project/commit/070315d04c6b which moved
`LLVM_SUPPORT_XCODE_SIGNPOSTS` from llvm-config.h.cmake to
config.h.cmake.

Also adds a comment for `HAVE_PROC_PID_RUSAGE`, that I spotted was
missing.

Differential Revision: https://reviews.llvm.org/D111569