[Triple] Add isThumb and isARM functions.

Summary:
isThumb returns true for Thumb triples (little and big endian), isARM
returns true for ARM triples (little and big endian).
There are a few more checks using arm/thumb that are not covered by
those functions, e.g. that the architecture is either ARM or Thumb
(little endian) or ARM/Thumb little endian only.

Reviewers: javed.absar, rengolin, kristof.beyls, t.p.northover

Reviewed By: rengolin

Subscribers: llvm-commits, aemerson

Differential Revision: https://reviews.llvm.org/D34682

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310781 91177308-0d34-0410-b5e6-96231b3b80d8