bsd-user: Implement RISC-V signal trampoline setup functions

bsd-user: Implement RISC-V signal trampoline setup functions

Added functions for setting up the RISC-V signal trampoline and signal
frame:

'set_sigtramp_args()': Configures the RISC-V CPU state with arguments
for the signal handler. It sets up the registers with the signal
number,pointers to the signal info and user context, the signal handler
address, and the signal frame pointer.

'setup_sigframe_arch()': Initializes the signal frame with the current
machine context.This function copies the context from the CPU state to
the signal frame, preparing it for the signal handler.

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
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: <20240916155119.14610-15-itachis@FreeBSD.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>