bsd-user:Add AArch64 improvements and signal handling functions

bsd-user:Add AArch64 improvements and signal handling functions

Added get_ucontext_sigreturn function to check processor state ensuring current execution mode is EL0 and no flags
indicating interrupts or exceptions are set.
Updated AArch64 code to use CF directly without reading/writing the entire processor state, improving efficiency.
Changed FP data structures to use Int128 instead of __uint128_t, leveraging QEMU's generic mechanism for referencing this type.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Ajeet Singh <itachis@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20240707191128.10509-9-itachis@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>