aarch64: testsuite: symbol-range fallback to compile

aarch64: testsuite: symbol-range fallback to compile

On some of our embedded aarch64 targets, RAM size is too small for
this test to fit.  It doesn't look like this test requires linking,
and if it does, the -tiny version may presumably get most of the
coverage without going overboard in target system requirements.

Still, linking may be useful, so introduce a two_plus_gigs effective
target, that checks for the ability to link a program with 2GiB of
sbss, and use that to select whether to link or just compile
symbol-range.c.

for  gcc/ChangeLog

	* doc/sourcebuild.texi (Environment attributes): Document
	two_plus_gigs.

for  gcc/testsuite/ChangeLog

	* lib/target-supports.exp
	(check_effective_target_two_plus_gigs): New.
	* gcc.target/aarch64/symbol-range.c: Link only on
	two_plus_gigs targets, compile otherwise.