libstdc++: Build libstdc++_libbacktrace.a as PIC [PR111936]

libstdc++: Build libstdc++_libbacktrace.a as PIC [PR111936]

In order for std::stacktrace to be used in a shared library, the
libbacktrace symbols need to be built with -fPIC. Add the libtool
-prefer-pic flag to the commands in src/libbacktrace/Makefile so that
the archive contains PIC objects.

libstdc++-v3/ChangeLog:

	PR libstdc++/111936
	* src/libbacktrace/Makefile.am: Add -prefer-pic to libtool
	compile commands.
	* src/libbacktrace/Makefile.in: Regenerate.