Create a constant pool symbol for the GOT in the ARMCGBR the same way we
do in the SDag when lowering references to the GOT: use
ARMConstantPoolSymbol rather than creating a dummy global variable. The
computation of the alignment still feels weird (it uses IR types and
datalayout) but it preserves the exact previous behavior. This change
fixes the memory leak of the global variable detected on the valgrind
leak checking bot.

Thanks to Benjamin Kramer for pointing me at ARMConstantPoolSymbol to
handle this use case.