Fix uc_mcontext register access on 32-bit PowerPC (#898)

Fixes #894

absl/debugging/internal/examine_stack.cc

@@ -56,7 +56,7 @@ void* GetProgramCounter(void* vuc) {
 #elif defined(__powerpc64__)
     return reinterpret_cast<void*>(context->uc_mcontext.gp_regs[32]);
 #elif defined(__powerpc__)
-    return reinterpret_cast<void*>(context->uc_mcontext.regs->nip);
+    return reinterpret_cast<void*>(context->uc_mcontext.uc_regs->gregs[32]);
 #elif defined(__riscv)
     return reinterpret_cast<void*>(context->uc_mcontext.__gregs[REG_PC]);
 #elif defined(__s390__) && !defined(__s390x__)

absl/debugging/internal/stacktrace_powerpc-inl.inc

@@ -131,7 +131,11 @@ static void **NextStackFrame(void **old_sp, const void *uc) {
       const ucontext_t* signal_context =
           reinterpret_cast<const ucontext_t*>(uc);
       void **const sp_before_signal =
+#if defined(__PPC64__)
           reinterpret_cast<void**>(signal_context->uc_mcontext.gp_regs[PT_R1]);
+#else
+          reinterpret_cast<void**>(signal_context->uc_mcontext.uc_regs->gregs[PT_R1]);
+#endif
       // Check that alleged sp before signal is nonnull and is reasonably
       // aligned.
       if (sp_before_signal != nullptr &&