eclass/tests/toolchain-funcs.sh: fix gold tests, bug #700812
As written tests assumed that $CC always called ld.bfd as a linker.
The assumption fails when default linker is not ld.bfd,
(for example sys-devel/binutils[default-gold]).
The change uses LDFLAGS=-fuse-ld=bfd explicitly.
Fixes the following tests:
* Testing tc-ld-is-gold (bfd selected) ... [ !! ]
* Testing tc-ld-disable-gold (bfd selected) ... [ !! ]
Reported-by: Michał Górny
Closes: https://bugs.gentoo.org/700812
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>