The "save" routines from the kernel, which are currently
commented out, are unnecessary in qemu. We can copy from
env where the kernel needs special instructions.
Drop the return value from restore_sigregs, as it cannot fail.
Use __get_user return as input to trace, so that we properly
bswap the value for the host.
Reorder the function bodies to correspond to the kernel source.
Fix the use of host addresses where guest addresses are needed.
Drop the use of PSW_ADDR_AMODE, since we only support 64-bit.
Set psw.mask properly for the signal handler.
Use tswap_sigset in setup_rt_frame.