target/ppc: Move 601 hflags adjustment to hreg_compute_hflags

target/ppc: Move 601 hflags adjustment to hreg_compute_hflags

Keep all hflags computation in one place, as this will be
especially important later.

Introduce a new POWERPC_FLAG_HID0_LE bit to indicate when
LE should be taken from HID0.  This appears to be set if
and only if POWERPC_FLAG_RTC_CLK is set, but we're not
short of bits and having both names will avoid confusion.

Note that this was the only user of hflags_nmsr, so we can
perform a straight assignment rather than mask and set.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210315184615.1985590-3-richard.henderson@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>