Include what you use in AArch64AsmBackend.cpp

Include what you use in AArch64AsmBackend.cpp

AArch64AsmBackend.cpp was not using any APIs from AArch64.h, and was
only including it for transitive dependencies.  Doing so is problematic
from include-what-you-use perspective, but it is also a layering issue
(it creates a dependency cycle between the primary AArch64 target
library and the MCTargetDesc library).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361774 91177308-0d34-0410-b5e6-96231b3b80d8