[ARM] Make sure GlobalISel is only initialized once. NFCI

Move some code inside the proper 'if' block to make sure it is only run once,
when the subtarget is first created. Things can still break if we use different
ARM target machines or if we have functions with different 'target-cpu' or
'target-features', we should fix that too in the future.

llvm-svn: 286974