[bootstrap] Allow passing options to sub-builds for all targets

This patch makes it possible to pass a CMake option to one of the runtimes
for all targets being built. Basically, any option that starts with the
name of a runtime project being built will be forwarded as-is to the
sub-build. This is useful for customizing a sub-build for all targets.

Differential Revision: https://reviews.llvm.org/D121822