Add a TargetOption to enable/disable GlobalISel

Summary:
This patch adds a new target option in order to control GlobalISel.
This will allow the users to enable/disable GlobalISel prior to the
backend by calling `TargetMachine::setGlobalISel(bool Enable)`.

No test case as there is already a test to check GlobalISel
command line options.
See: CodeGen/AArch64/GlobalISel/gisel-commandline-option.ll.

Reviewers: qcolombet, aemerson, ab, dsanders

Reviewed By: qcolombet

Subscribers: rovka, javed.absar, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D42137

llvm-svn: 322773