[BOLT] Add BOLT_TARGETS_TO_BUILD

Adds BOLT_TARGETS_TO_BUILD, which defaults to the intersection of
X86;AArch64 and LLVM_TARGETS_TO_BUILD, but allows configuration to
alter that -- for instance omitting one of those two targets even if
llvm supports both.

Reviewed By: rafauler

Differential Revision: https://reviews.llvm.org/D148847