FreeBSD_HEAD_i386 - Build #4122 - Fixed

FreeBSD_HEAD_i386 - Build #4122 - Fixed:

Build information: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_i386/4122/
Full change log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_i386/4122/changes
Full build log: https://jenkins.FreeBSD.org/job/FreeBSD_HEAD_i386/4122/console

Change summaries:

307700 by ngie:
Only build lib/libc/tests/iconv if MK_ICONV != no

MFC after:	1 week
Reported by:	damian_at_damianek.be
Sponsored by:	Dell EMC Isilon

307699 by ngie:
Revert r307689

The proposed change ("Fix building of llvm's unwind if gcc has been
also built") breaks the build with clang/llvm.

Tested with...

	(
	export SRCCONF=/dev/null WITH_CLANG=
	cd gnu/lib/libgcc; make obj; make depend; make all
	)

MFC after:	3 days
X-MFC with:	r307689
Pointyhat to:	bapt
Reported by:	Jenkins, O. Hartmann <ohartman_at_zedat.fu-berlin.de>
Sponsored by:	Dell EMC Isilon

307698 by lwhsu:
- Add required header for fixing `make` in sys/modules/gpio

Reviewed by:	imp, loos
Differential Revision:	https://reviews.freebsd.org/D7815