build: Fix warning about linking with a non-POSIX ar.
build: Fix warning about linking with a non-POSIX ar.

```
ltlibrary.am: warning: 'libunwind-sh.la': linking libtool libraries using a non-POSIX
ltlibrary.am: archiver requires 'AM_PROG_AR' in 'configure.ac'
src/Makefile.am:622:   while processing Libtool library 'libunwind-sh.la'
```

unkadoug/libunwind,vtjnash/libunwind,libunwind/libunwind,yuyichao/libunwind,yuyichao/libunwind,djwatson/libunwind,vtjnash/libunwind,libunwind/libunwind,frida/libunwind,vtjnash/libunwind,rantala/libunwind,frida/libunwind,unkadoug/libunwind,rantala/libunwind,libunwind/libunwind,rantala/libunwind,djwatson/libunwind,unkadoug/libunwind,djwatson/libunwind,frida/libunwind,yuyichao/libunwind