Fix ld tests with sysroot=/ and --enable-targets=all and test --print-sysroot

Fix ld tests with sysroot=/ and --enable-targets=all and test --print-sysroot

	* ld-scripts/sysroot-prefix.exp: Log $ld_sysroot.  Handle sysroot
	== "/" as a separate sysroot-configuration with separable
	test-types.
	(sysroot_prefix_tests): Include all existing sysroot tests in
	sysroot == "/" tests except exclude those where a --sysroot option
	is not specified.
	* lib/ld-lib.exp (check_sysroot_available): Rewrite to use
	--print-sysroot instead of relying on error code from using
	--sysroot=...  Also, set $ld_sysroot.

The reason we exclude not just the failing "full-path =-prefixed
without" but also the passing "plain =-prefixed without but -Lpath"
for sysroot == "/" is that for the latter to succeed, we have to make
assumptions about the system not having a /sysroot directory or
assumptions about its contents etc.

When passing --enable-targets=all --enable-64-bit-bfd (the
latter not required for a "64-bit-host" of course) the ld --help
output got too much to handle for poor tcl (or maybe dejagnu is
to blame) and remote_exec exited with an error, so the
configuration being tested was mishandled as being a
sysroot-less configuration.  Using --version instead of --help
would work too, but the new --print-sysroot option calls for
nominal coverage, so why not use that instead.