bsd-user/arm/target_arch_cpu.h: Implement data abort exceptions

bsd-user/arm/target_arch_cpu.h: Implement data abort exceptions

Implement EXCP_PREFETCH_ABORT AND EXCP_DATA_ABORT. Both of these data
exceptions cause a SIGSEGV.

Signed-off-by: Kyle Evans <kevans@FreeBSD.org>
Signed-off-by: Olivier Houchard <cognet@ci0.org>
Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>