[unwind] fix build with GCC on PPC32

Originally reported downstream in Gentoo: https://bugs.gentoo.org/832140



```
/var/tmp/portage/sys-libs/llvm-libunwind-13.0.0/work/libunwind/src/libunwind.cpp:77:3: error: #error Architecture not supported
   77 | # error Architecture not supported
      |   ^~~~~
[...]
/var/tmp/portage/sys-libs/llvm-libunwind-13.0.0/work/libunwind/src/libunwind.cpp: In function ‘int __unw_init_local(unw_cursor_t*, unw_context_t*)’:
/var/tmp/portage/sys-libs/llvm-libunwind-13.0.0/work/libunwind/src/libunwind.cpp:80:57: error: ‘REGISTER_KIND’ was not declared in this scope
   80 |   new (reinterpret_cast<UnwindCursor<LocalAddressSpace, REGISTER_KIND> *>(cursor))
      |                                                         ^~~~~~~~~~~~~
[...]
```

PPC is actually a supported architecture, but GCC (tested with 11.2.0)
on powerpc32 seems to only define: `__PPC__, _ARCH_PPC, __PPC,
__powerpc` and //not// `__ppc__`.

This instead uses `__powerpc__` which should be around on PPC32
and PPC64 (but we check it after PPC64, so it's fine).

Signed-off-by: Sam James <sam@gentoo.org>
Differential Revision: https://reviews.llvm.org/D118320