Remove yet another buried and hidden implicit dependency: every single

sub-library for the targets depended on the core target CodeGen library.
This completely undermined the careful work to separate the those
libraries, especially the MC-layer ones. This surfaced as circular
dependencies when the libraries were built as shared libraries where
CMake doesn't allow cycles.

This should fix PR10537. I'll watch the bots to see if there is fallout
on other platforms.

llvm-svn: 136565