[PowerPC] [patch] mtmsr does not preserve high bits of MSR

The mtmsr instruction on 64-bit PPC does not preserve the high-order 32-bits of the MSR the way it is supposed to, instead setting them to 0, which takes 64-bit code out of 64-bit mode. There is some code that does the right thing, but it brokenly only preserves these bits when the thread is not in 64-bit mode (i.e. when it doesn't matter). The attached patch unconditionally enables this code when TARGET_PPC64 is set, per the ISA spec, which fixes early boot failures trying to start FreeBSD/powerpc64 under qemu.