Literal movement code may grow auto litpool so big that it won't be
possible to jump around it. Limit the size of auto litpools by 1/2 of
the jump range.

gas/
2018-02-20  Max Filippov  <jcmvbkbc@gmail.com>

	* config/tc-xtensa.c (struct litpool_frag): Add new field
	literal_count.
	(MAX_AUTO_POOL_LITERALS, MAX_EXPLICIT_POOL_LITERALS)
	(MAX_POOL_LITERALS): New macro definitions.
	(auto_litpool_limit): Initialize to 0.
	(md_parse_option): Set auto_litpool_limit in the presence of
	--auto-litpools option.
	(xtensa_maybe_create_literal_pool_frag): Zero-initialize
	literal_count field.
	(xg_find_litpool): New function. Make sure that found literal
	pool size is within the limit.
	(xtensa_move_literals): Extract literal pool search code into
	the new function.
	* testsuite/gas/xtensa/all.exp: Add auto-litpools-2 test.
	* testsuite/gas/xtensa/auto-litpools-2.d: New file.
	* testsuite/gas/xtensa/auto-litpools-2.s: New file.
	* testsuite/gas/xtensa/auto-litpools.d: Fix up changed
	addresses.
	* testsuite/gas/xtensa/auto-litpools.s: Change literal value so
	that objdump doesn't get out of sync.