fix fpregset_t type on powerpc64

the userspace ucontext API has this as an array rather than a
structure.

commit 3c59a868 fixed the
corresponding mistake for vrregset_t, namely that the original
powerpc64 port used a mix of types from 32-bit powerpc and powerpc64
rather than matching the 64-bit types.

arch/powerpc64/bits/signal.h

@@ -9,11 +9,7 @@
 #if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
 
 typedef unsigned long greg_t, gregset_t[48];
-
-typedef struct {
-	double fpregs[32];
-	double fpscr;
-} fpregset_t;
+typedef double fpregset_t[33];
 
 typedef struct {
 #ifdef __GNUC__