bsd-user:Add ARM AArch64 signal handling support

bsd-user:Add ARM AArch64 signal handling support

Added sigcode setup function for signal trampoline which initializes a sequence of instructions
to handle signal returns and exits, copying this code to the target offset.
Defined ARM AArch64 specific signal definitions including register indices and sizes,
and introduced structures to represent general purpose registers, floating point registers, and machine context.
Added function to set up signal handler arguments, populating register values in `CPUARMState`
based on the provided signal, signal frame, signal action, and frame address.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Co-authored-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240707191128.10509-5-itachis@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>