Summary:
The default behavior for LTO on other targets does not specify the
number of LTO partitions. Recent changes made this default to 8 on
AMDGPU which had some issues with the `libc` project. The option to
disable this is HIP only so I think for now we should restrict this just
to HIP.
I'm definitely on board with getting some more parallelism here, but I
think it should probably be restricted to just offloading languages. The
new driver goes through the `--target=amdgcn-amd-amdhsa` for its output,
which means we'd need to forward the default somehow.
d0aa1f9c43