[libunwind] Further fix for 32-bit PowerPC processors without AltiVec

https://reviews.llvm.org/D91906 did most of the work necessary to fix libunwind on
32-bit PowerPC processors without AltiVec, but there was one more piece necessary.

Reviewed By: luporl

Differential Revision: https://reviews.llvm.org/D120197

(cherry picked from commit 3fa2e66)

Author: kernigh

libunwind/src/UnwindRegistersSave.S

@@ -603,9 +603,11 @@ DEFINE_LIBUNWIND_FUNCTION(__unw_getcontext)
   stw     30,128(3)
   stw     31,132(3)
 
+#if defined(__ALTIVEC__)
   // save VRSave register
   mfspr   0, 256
   stw     0, 156(3)
+#endif
   // save CR registers
   mfcr    0
   stw     0, 136(3)