target/i386: only include bits in pg_mode if they are not ignored

LA57/PKE/PKS is only relevant in 64-bit mode, and NXE is only relevant if
PAE is in use.  Since there is code that checks PG_MODE_LA57 to determine
the canonicality of addresses, make sure that the bit is not set by
mistake in 32-bit mode.  While it would not be a problem because 32-bit
addresses by definition fit in both 48-bit and 57-bit address spaces,
it is nicer if get_pg_mode() actually returns whether a feature is enabled,
and it allows a few simplifications in the page table walker.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>