[libc] Do not install to the default triple in GPU mode

Currently this logic causes the `libcgpu.a` to be installed to the
user's default triple. The GPU target isn't really true to this in a
cross-compiling sense so we should just default to the regular `/lib`
directory

Reviewed By: sivachandra, lntue

Differential Revision: https://reviews.llvm.org/D142774