Summary:
Right now it's a little difficult to use the multilibs support because
the user must manually provide one. I believe that when the user
configures multilibs with the LLVM CMake arguments at a minimum we
should provide one that forward `-fmultilib-flag=<multilib>` to the
created runtime.
This RP makes CMake emit this by manually writing a flag. Because users
could provide their own, this adds some extre complexity to prevent this
from being overwritten.
The desire for this change is to more easily ship this support in CMake
configuration files without needing to write files manually (for the
typical case).
b49383c8bb