linux-user: Minimum Sig Handler Stack Size for PPC64 ELF V2

The ELF V2 ABI for PPC64 defines MINSIGSTKSZ as 4096 bytes whereas it was
2048 previously.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/sparc/syscall.h

@@ -15,3 +15,4 @@ struct target_pt_regs {
  * and copy_thread().
  */
 #define TARGET_CLONE_BACKWARDS
+#define TARGET_MINSIGSTKSZ      4096