x86_64, x32: implement rt_sigreturn decoding

* syscall.c [X86_64 || X32] (x86_64_rsp_ptr): New variable.
* linux/x86_64/arch_regs.h (x86_64_rsp_ptr): New declaration.
* linux/x86_64/syscallent.h (15): Use sys_sigreturn for rt_sigreturn
decoding.
* linux/x32/syscallent.h (513): Likewise.
* signal.c (sys_sigreturn) [X86_64 || X32]: Print signal mask
for non-i386 personalities.
5 files changed