[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319620
91177308-0d34-0410-b5e6-
96231b3b80d8