[CMake] Support multi-target runtimes build

This changes adds support for building runtimes for multiple
different targets using LLVM runtimes directory.

The implementation follow the model used already by the builtins
build which already supports this option. To specify the runtimes
targets to be built, use the LLVM_RUNTIME_TARGETS variable, where
the valuae is the list of targets to build runtimes for. To pass
a per target variable to the runtimes build, you can set
RUNTIMES_<target>_<variable> where <variable> will be passed to the
runtimes build for <target>.

Each runtime target (except for the default one) will be installed
into lib/<target> subdirectory. Build targets will be suffixed with
the target name.

Differential Revision: https://reviews.llvm.org/D32816

llvm-svn: 307731