Revert "[ARM] Support constant pools in data when generating execute-only code."

This reverts commit 3a204faa093c681a1e96c5e0622f50649b761ee0.

I've upset a buildbot which runs the address sanitizer:
ERROR: AddressSanitizer: stack-use-after-scope
lib/Target/ARM/ARMISelLowering.cpp:2690
That Twine variable is used illegally.

llvm-svn: 305390