[compiler-rt] Remove out of date comment
Per beanz, building compiler-rt standalone is a pretty important use
case, so the comment is very out of date.
# CMake build for CompilerRT.
#
-# This build assumes that CompilerRT is checked out into the
-# 'projects/compiler-rt' or 'runtimes/compiler-rt' inside of an LLVM tree.
-# Standalone build system for CompilerRT is not yet ready.
-#
# An important constraint of the build is that it only produces libraries
# based on the ability of the host toolchain to target various platforms.
