[libcxx] Use interface library for libcxx-abi-shared

The libc++.so linker script generation uses the IMPORTED_LIBNAME
target property on libcxx-abi-shared. However, libcxx-abi-shared
is not an interface library and as such cannot have an
IMPORTED_LIBNAME target property.

Convert libcxx-abi-shared into an imported interface library
and use IMPORTED_LIBNAME in place of IMPORTED_LOCATION. This makes
linker script generation work correctly with system-libcxxabi.

I believe this fixes the issue that D131037 was intended to fix.

Differential Revision: https://reviews.llvm.org/D133566