libc: fix missing linux-raw-dep

[HmnSn]

After #1478, Rustix supports building without linux-raw-sys on Linux but missing some architectures. This causes building Mesa3D failed on mips linux:

error[E0433]: failed to resolve: use of unresolved module or unlinked crate `linux_raw_sys`
  --> src/backend/libc/c.rs:80:34
   |
80 | pub(crate) const SIGEMT: c_int = linux_raw_sys::general::SIGEMT as _;
   |                                  ^^^^^^^^^^^^^ use of unresolved module or unlinked crate `linux_raw_sys`
   |
   =  

This PR extends the support to cover it.