linux-user/ppc: Fix msr updates for signal handling

linux-user/ppc: Fix msr updates for signal handling

In save_user_regs, there are two bugs where we OR in a bit number
instead of the bit, clobbering the low bits of MSR.  However:

The MSR_VR and MSR_SPE bits control the availability of the insns.
If the bits were not already set in MSR, then any attempt to access
those registers would result in SIGILL.

For linux-user, we always initialize MSR to the capabilities
of the cpu.  We *could* add checks vs MSR where we currently
check insn_flags and insn_flags2, but we know they match.

Also, there's a stray cut-and-paste comment in restore.

Then, do not force little-endian binaries into big-endian mode.

Finally, use ppc_store_msr for the update to affect hflags.
Which is the reason none of these bugs were previously noticed.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210323184340.619757-10-richard.henderson@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>