[libunwind] Define _Unwind_Backtrace for powerpc, sparc

[thesamesam]

Summary

Add SPARC to the list of platforms for which we provide a full
unwind implementation which leads to _Unwind_Backtrace being defined within
libunwind.so.

Likewise for PPC (see D118320 for background).

[Arfrever]

During investigation for D118320, it was found that:

• GCC on PPC32 defines __powerpc__, but not __ppc__ (Clang on PPC32 defines both)
• GCC on PPC64 defines both __powerpc__ and __powerpc64__ (Clang on PPC64 defines both)

So problem with functions such as _Unwind_Backtrace not being available is probably also present in libunwind built by GCC on PPC32.
Then defined(__ppc__) || defined(__ppc64__) || defined(__powerpc64__) could be merged into simple defined(__powerpc__).

[thesamesam]

Fix similar issue for PPC32.

[MaskRay]

LGTM.

[koakuma]

Looks okay here too check-unwind tests passes and _Unwind_Backtrace seems to work on Gentoo/sparc64, at least.

[100%] Running libunwind tests
-- Testing: 11 tests, 11 workers --
Testing:  0.. 10.. 20.. 30.. 40.. 50.. 60.. 70.. 80.. 90..

Testing Time: 1.82s
  Unsupported: 4
  Passed     : 7
[100%] Built target check-unwind

libunwind/src/config.h

 #if (defined(__APPLE__) && defined(__arm__)) || defined(__USING_SJLJ_EXCEPTIONS__)
 #define _LIBUNWIND_BUILD_SJLJ_APIS
 #endif
 
 #if defined(__i386__) || defined(__x86_64__) || defined(__powerpc__)
 #define _LIBUNWIND_SUPPORT_FRAME_APIS
 #endif
 
-#if defined(__i386__) || defined(__x86_64__) ||                                \
-    defined(__ppc__) || defined(__ppc64__) || defined(__powerpc64__) ||        \
-    (!defined(__APPLE__) && defined(__arm__)) ||                               \
-    defined(__aarch64__) ||                                                    \
-    defined(__mips__) ||                                                       \
-    defined(__riscv) ||                                                        \
-    defined(__hexagon__)
+#if defined(__i386__) || defined(__x86_64__) || defined(__ppc__) ||            \
+    defined(__ppc64__) || defined(__powerpc64__) ||                            \
+    (!defined(__APPLE__) && defined(__arm__)) || defined(__aarch64__) ||       \
+    defined(__mips__) || defined(__riscv) || defined(__hexagon__) ||           \
+    defined(__sparc__)
 #if !defined(_LIBUNWIND_BUILD_SJLJ_APIS)
 #define _LIBUNWIND_BUILD_ZERO_COST_APIS
 #endif
 #endif
 
 #ifndef _LIBUNWIND_REMEMBER_HEAP_ALLOC
 #if defined(_LIBUNWIND_REMEMBER_STACK_ALLOC) || defined(__APPLE__) ||          \
     defined(__linux__) || defined(__ANDROID__) || defined(__MINGW32__) ||      \