gn build: Simplify Target build files

Now that the cycle between MCTargetDesc and TargetInfo is gone
(see revisions 360709 360718 360722 360724 360726 360731 360733 360735 360736),
remove the dependency from TargetInfo on MCTargetDesc:tablegen. In most
targets, this makes MCTargetDesc:tablegen have just a single use, so
inline it there.

For AArch64, ARM, and RISCV there's still a similar cycle between
MCTargetDesc and Utils, so the MCTargetDesc:tablegen indirection is
still needed there.

Differential Revision: https://reviews.llvm.org/D63200

llvm-svn: 363436