[CMake] Multi-target builtins build

This change enables building builtins for multiple different targets
using LLVM runtimes directory.

To specify the builtin targets to be built, use the LLVM_BUILTIN_TARGETS
variable, where the value is the list of targets.  To pass a per target
variable to the builtin build, you can set BUILTINS_<target>_<variable>
where <variable> will be passed to the builtin build for <target>.

Differential Revision: https://reviews.llvm.org/D26652

llvm-svn: 289491