[testsuite] note pitfall in how outputs.exp sets gld

This patch documents a glitch in gcc.misc-tests/outputs.exp: it checks
whether the linker is GNU ld, and uses that to decide whether to
expect collect2 to create .ld1_args files under -save-temps, but
collect2 bases that decision on whether HAVE_GNU_LD is set, which may
be false zero if the linker in use is GNU ld.  Configuring
--with-gnu-ld fixes this misalignment.  Without that, atsave tests are
likely to fail, because without HAVE_GNU_LD, collect2 won't use @file
syntax to run the linker (so it won't create .ld1_args files).

Long version: HAVE_GNU_LD is set when (i) DEFAULT_LINKER is set during
configure, pointing at GNU ld; (ii) --with-gnu-ld is passed to
configure; or (iii) config.gcc sets gnu_ld=yes.  If a port doesn't set
gnu_ld, and the toolchain isn't configured so as to assume GNU ld,
configure and thus collect2 conservatively assume the linker doesn't
support @file arguments.

But outputs.exp can't see how configure set HAVE_GNU_LD (it may be
used to test an installed compiler), and upon finding that the linker
used by the compiler is GNU ld, it will expect collect2 to use @file
arguments when running the linker.  If that assumption doesn't hold,
atsave tests will fail.


for  gcc/testsuite/ChangeLog

	* gcc.misc-tests/outputs.exp (gld): Note a known mismatch and
	record a workaround.

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