[CMake][runtimes] Don't passthrough prefixes for non-default targets

The passthrough is useful for setting up the options for the default
build, but we already have a different mechanism to pass CMake flags
to builds for builtins and runtimes targets so this is not really
needed there. Furthermore, when the flags are set for the default
build, with the prefix passthrough set we have to explicitly override
all options in other targets which can be cumbersome.

Differential Revision: https://reviews.llvm.org/D39988

llvm-svn: 318571