target-alpha: Fixes for alpha-linux syscalls.

1. Add correct definitions of error numbers.
2. Implement SYS_osf_sigprocmask
3. Implement SYS_osf_get/setsysinfo for IEEE_FP_CONTROL.

This last requires exposing the FPCR value to do_syscall.
Since this value is actually split up into the float_status,
expose routines from helper.c to access it.

Finally, also add a float_exception_mask field to float_status.
We don't actually use it to control delivery of exceptions to
the emulator yet, but simply hold the value that we placed there
when loading/storing the FPCR.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
