Place pkg-config file in $prefix/share/pkgconfig.

libclc is ABI-agnostic, and $prefix/lib/pkgconfig causes issues
on multilib setups. Using $prefix/share/pkgconfig allows us to reuse
a single libclc build across all system ABIs.

Patch by: Michał Górny

git-svn-id: https://llvm.org/svn/llvm-project/libclc/trunk@190107 91177308-0d34-0410-b5e6-96231b3b80d8