[ARM][NFC] Avoid recreating MCSubtargetInfo in ARMAsmBackend

After D41349, we can now directly access MCSubtargetInfo from 
createARM*AsmBackend. This patch makes use of this, avoiding the need to 
create a fresh MCSubtargetInfo (which was previously always done with a blank 
CPU and feature string). Given the total size of the change remains pretty 
tiny and we're removing the old explicit destructor, I changed the STI field 
to a reference rather than a pointer.

Differential Revision: https://reviews.llvm.org/D41693

llvm-svn: 321707