[AArch64] [Assembler] option to disable negative immediate conversions

Summary:
Similar to the ARM target in https://reviews.llvm.org/rL298380, this
patch adds identical infrastructure for disabling negative immediate
conversions, and converts the existing aliases to the new infrastucture.

Reviewers: rengolin, javed.absar, olista01, SjoerdMeijer, samparker

Reviewed By: samparker

Subscribers: samparker, aemerson, llvm-commits

Differential Revision: https://reviews.llvm.org/D31243

llvm-svn: 298908