Use general instruction/stack pointer convenience method instead of manually

src/google_breakpad/common/minidump_cpu_ppc64.h

Index: src/google_breakpad/common/minidump_cpu_ppc64.h
===================================================================
--- src/google_breakpad/common/minidump_cpu_ppc64.h	(revision 1466)
+++ src/google_breakpad/common/minidump_cpu_ppc64.h	(working copy)
@@ -112,6 +112,11 @@
   MDVectorSaveAreaPPC   vector_save;
 } MDRawContextPPC64;  /* Based on ppc_thread_state */
 
+/* Indices into gpr for registers with a dedicated or conventional purpose. */
+enum MDPPC64RegisterNumbers {
+  MD_CONTEXT_PPC64_REG_SP = 1
+};
+
 /* For (MDRawContextPPC).context_flags.  These values indicate the type of
  * context stored in the structure.  MD_CONTEXT_PPC is Breakpad-defined.  Its
  * value was chosen to avoid likely conflicts with MD_CONTEXT_* for other