[cmake] Move CMAKE_(C|CXX)_COMPILER variables before CROSS_TOOLCHAIN_FLAGS so

they can be overridden when cross compiling.

Summary:
Since CROSS_TOOLCHAN_FLAGS can set CMAKE_(C|CXX)_COMPILER
variables, move the compiler variables up front so they can be
overridden.

This is a followup to https://reviews.llvm.org/D40229 committed in rL319620.

Thanks to Pavel Labath for reporting this issue.

Reviewers: labath, beanz

Subscribers: mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D40896

llvm-svn: 319898