[Hexagon] Make memcpy lowering thread-safe

[Hexagon] Make memcpy lowering thread-safe

This removes an unpleasant hack involving a global variable for special
lowering of certain memcpy calls. These are now lowered as intended in
EmitTargetCodeForMemcpy in the same way that other targets do it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255785 91177308-0d34-0410-b5e6-96231b3b80d8