[cmake] Re-commit: Remove redundant call to cmake when building host tools.

Also pass CMAKE_(C|CXX)_COMPILER to add_custom_command.

Summary:
Remove the redundant, config-time call to cmake when
building host tools for cross compiles or optimized tablegen..

The config-time call to cmake is redundant because it will always get
called again when the CONFIGURE_LLVM_${target_name} target fires at
build-time.  This speeds up initial configuration, but has no affect
on build behavior.

Differential Revision: https://reviews.llvm.org/D40229

llvm-svn: 319620