If CONFIG_BUILD_ELF64 was not selected and gcc had -msym32 option
(i.e. 4.0 or newer), there is no point to use %highest, %higher for
kernel symbols.

This patch also fixes 64-bit SMTC version of get_saved_sp() which is
broken but harmless since there is no such CPUs for now.

A bonus is set_saved_sp() and SMP version of get_saved_sp() are more
readable now.
Signed-off-by: NAtsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>