bootstrap/119680 - fix cross-compiler build with --enable-host-shared

bootstrap/119680 - fix cross-compiler build with --enable-host-shared

It seems that at least when cross-compiling at least collect2 pulls
in objects from libbacktrace.a which is linked via LIBDEPS.  But
libbacktrace for the host is only built -fPIC with --enable-host-shared
but not -fPIE with --enable-host-pie so this fails.  The following
teaches libbacktrace about --enable-host-pie and handles it similar
to libcpp.

	PR bootstrap/119680
libbacktrace/
	* configure.ac (--enable-host-pie): Handle by setting PIC_FLAG
	to -fPIE.
	* configure: Regenerate.