bsd-user: Implement target_set_brk function in bsd-mem.c instead of os-syscall.c

bsd-user: Implement target_set_brk function in bsd-mem.c instead of os-syscall.c

The definitions and variables names matches the corresponding ones in
linux-user/syscall.c, for making later implementation of do_obreak easier

Co-authored-by: Mikaël Urankar <mikael.urankar@gmail.com>
Signed-off-by: Mikaël Urankar <mikael.urankar@gmail.com>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230925182709.4834-9-kariem.taha2.7@gmail.com>