[libunwind][AIX] implementation of the unwinder for AIX

Summary:
This is an add-on patch to address comments.
- Replace #elif in file <assembly.h> with #else as suggested;
- Reversed the indentation changes in the main patch.

Differential Revision: https://reviews.llvm.org/D100132

libunwind/src/assembly.h

@@ -209,7 +209,7 @@
 #if defined(__powerpc64__)
 #define VBYTE_LEN 8
 #define CSECT_ALIGN 3
-#elif defined(__ppc__)
+#else
 #define VBYTE_LEN 4
 #define CSECT_ALIGN 2
 #endif