[libc] Only enable `LLVM_FULL_BUILD_MODE` by default for GPU targets (#84664)

Summary:
Currently we have a conditional that turns the full build on by default
if it is a default target. This used to work fine when the GPU was the
only target that was ever present. However, we've recently changed to
allow building multiple of these at the same time. That means we should
have the ability to build overlay mode in the CPU mode and full build in
the GPU mode. This patch makes some simple adjustments to pass the
arguments per-triple. This slightly extends the existing `-DRUNTIMES_`
argument support to also transform any extra CMake inputs rather than
just the passed CMake variables.