[gn build] Add build files for Target/X86/... and for tools/llc

The tablegen setup for Target/X86 is a bit different from the CMake build: In
the CMake build, Target/X86/CMakeLists.txt has a single tablegen target that
does everything. But some of the generated files are only used privately by a
subproject, so in the GN build some of the tablegen invocations are
smaller-scoped, mostly for build cleanliness. (It helps also a tiny bit with
build parallelism since now e.g. the cpp files in MCTargetDesc can build after
just 3 .inc files are generated instead of being blocked on all 13. But it's
not a big win, since things depending on Target still need to wait for all 11,
even though all .inc file use is internal to lib/Target.)

Also add a build file for llc, since now all its dependencies have build files.

Differential Revision: https://reviews.llvm.org/D55524

llvm-svn: 348903