[CMake][runtimes] Use the same configuration for non-target and "default" target

[CMake][runtimes] Use the same configuration for non-target and "default" target

The default host target for builtins and runtimes has special behavior
on some platforms, e.g. on Linux both i386 and x86_64 targets are being
built. Specifying "default" as a target name should lead to the same
behavior, which wasn't the case in the past. This patch unifies the
configuration between the non-target and "default" target to produce the
same behavior by moving the default configuration into a function that
can be used from both paths.

Differential Revision: https://reviews.llvm.org/D37450

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312831 91177308-0d34-0410-b5e6-96231b3b80d8