lib/external/fexec.c: error out if chroot/chdir fails, ignore target_arch.
Ignore executing install/remove scripts if XBPS_TARGET_ARCH is set.

Instead a two stage approach is necessary when packages for target arch
are installed:

	- XBPS_TARGET_ARCH=arch xbps-install foo <- only unpack
	- xbps-reconfigure -a <- configure natively or via an emulator