sparc32: implement proper LEON support in head_32 (before highmem)

Use PSR to check if the CPU is LEON and jump to
LEON specific code in this case.

Added a few constants to psr.h to increase readability.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Daniel Hellstrom <daniel@gaisler.com>
Cc: Konrad Eisele <konrad@gaisler.com>