bsd-user/arm/target_arch_cpu.h: Implement system call dispatch

bsd-user/arm/target_arch_cpu.h: Implement system call dispatch

Implement the system call dispatch. This implements all three kinds of
system call: direct and the two indirect variants. It handles all the
special cases for thumb as well.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Kyle Evans <kevans@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>