Fix gregs usage on 32-bit powerpc (#8)

The old code was wrong as `uc_regs` is not a registers array,
but rather an `mcontext_t` pointer on glibc, therefore indexing
it goes way past the memory bounds. The actual registers array
is `gregs` inside the `mcontext_t` structure.

On non-glibc libcs (as well as inside the kernel definitions),
the structure is defined differently, with `uc_mcontext` being
an actual value member just like on ppc64, so we can use that.
On glibc/ppc32, `mcontext_t` is an union of `uc_regs` and `regs`
pointers, with the value of the member being a pointer to where
the real `uc_mcontext` field would be.

sigsegv.c

@@ -95,7 +95,11 @@ static void signal_segv(int signum, siginfo_t* info, void*ptr) {
     for(i = 0; i < NGREG; i++)
         a2j_error("reg[%02d]       = 0x" REGFORMAT, i,
 #if defined(__powerpc__) && !defined(__powerpc64__)
-                ucontext->uc_mcontext.uc_regs[i]
+#if defined(__GLIBC__)
+                ucontext->uc_mcontext.uc_regs->gregs[i]
+#else
+                ucontext->uc_mcontext.gregs[i]
+#endif
 #elif defined(__powerpc64__)
                 ucontext->uc_mcontext.gp_regs[i]
 #elif defined(__sparc__) && defined(__arch64__)