ARM: kprobes: make more tests conditional

The mls instruction is not available in ARMv6K or below, so we
should make the test conditional on at least ARMv7. ldrexd/strexd
are available in ARMv6K or ARMv7, which we can test by checking
the CONFIG_CPU_32v6K symbol.

/tmp/ccuMTZ8D.s: Assembler messages:
/tmp/ccuMTZ8D.s:22188: Error: selected processor does not support ARM mode `mls r0,r1,r2,r3'
/tmp/ccuMTZ8D.s:22222: Error: selected processor does not support ARM mode `mlshi r7,r8,r9,r10'
/tmp/ccuMTZ8D.s:22252: Error: selected processor does not support ARM mode `mls lr,r1,r2,r13'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jon Medhurst <tixy@yxit.co.uk>
Acked-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Leif Lindholm <leif.lindholm@arm.com>