tests: correct builtins test if built under -mthumb on ARM

The clear_cache and enable_execute_stack tests attempt to memcpy the definition
of a function into a buffer before executing the function.  The problem with
this approach is that on some targets (ARM with thumb mode compilation, MIPS
with MIPS16 codegen or uMIPS), you would use a pointer which is incorrect (it
would be off-by-one) due to the ISA selection being encoded into the address.
This ensures that the function address is retrieved correctly in all cases.

llvm-svn: 225215