[TextAPI] Prefix all architecture enums to fix the build on i386.
Summary: This changes the Architecture enum to use a prefix (AK_) to prevent the
preprocessor from replacing i386 with 1 when building llvm/clang for i386.
Reviewers: steven_wu, lhames, mstorsjo
Reviewed By: mstorsjo
Subscribers: hiraditya, jkorous, dexonsmith, llvm-commits
Differential Revision: https://reviews.llvm.org/D60241