Ion ARM cleanups: use BaselineFrameReg, not r11; remove dead write to r7.

This is in preparation for conditionally changing BaselineFrameReg to r7
if the ABI would use r11 in a way that conflicts with the monkeys, as it
does for the APCS frame pointer option.
Make JIT code avoid r11 on ARM if APCS frame pointers are in use.

Specifically: don't allocate r11 in IonMonkey, and use r7 instead for
the BC frame pointer (incompatible with the APCS frame format).

Additionally, make the trampolines preserve r7 in this case.