[libc++][libc++abi][libunwind] Dedup install path var definitions

In D116873 I did this for libunwind prior to defining a new install path
variable. But I think the change is good on its own, and libc++{,abi}
could also use it.

libc++ needed the base header var defined above the conditional part to
use it for the prefi+ed headers in the non-target-specific case. For
consistency, I therefore put the unconditional ones above for all 3
libs, which is why I touched the libunwind code (seeing that it had the
core change already)

Reviewed By: phosek, #libunwind, #libc, #libc_abi, ldionne

Differential Revision: https://reviews.llvm.org/D116988