[CMake][runtimes] Process common options in runtimes build

This was removed in D39932 but turned out this is actually needed
because runtimes such as compiler-rt and libc++ rely on common options
processing for setting certain flags such as -ffunction-sections and
-fdata-sections.

Differential Revision: https://reviews.llvm.org/D45507

llvm-svn: 329778