[PowerPC64LE] Work around qemu TCG bug in mtmsrd emulation.

The TCG implementation of mtmsrd in qemu blindly copies the entire register
to the MSR, instead of the specific bit positions listed in the ISA.

This means that qemu will prematurely switch endian out from under the
running code instead of waiting for the rfid, causing an immediate trap
as it attempts to interpret the next instruction in the wrong endianness.

To work around this, ensure PSL_LE is still set before doing the mtmsrd.

In the future, we may wish to just turn off translation and unconditionally
use rfid to switch to the ofmsr instead of quasi-switching to the ofmsr.

Add a new platform option so this can be disabled. (And so that we can
conditonalize additional QEMU-specific hacks in the platform code.)

Sponsored by: Tag1 Consulting, Inc.