1.0.25.54: centralize scattered arch_os_get_context() calls

1.0.25.54: centralize scattered arch_os_get_context() calls

... to the six signal handlers trampolines. These are now the only
places where void_context appears, the rest of the runtime uses
os_context_t.

Also, the &void_context+37 hack was removed. On Sparc/Linux the third
parameter of SA_SIGINFO signal handlers is a pointer to sigcontext,
which happens to be the same as &void_context+37 most of the time,
though.

I have tested two on Sparc/Linux boxes, one running 2.6.26 that
randomly segfaulted compiling itself with 1.0.25.12, and another
runnin 2.6.24 that worked fine before at that version.

Thanks to Bruce O'Neel for the shell access.