gas: arm -mwarn-syms duplicates

arm gas is only supposed to warn once per symbol for -mwarn-syms, but
doesn't because the str_hash_find added with commit 629310ab
always returns NULL.  That's so because the str_hash_insert inserts a
NULL value for the key,value pair.  Let str_hash_insert do the job
instead.

	* config/tc-arm.c (arm_tc_equal_in_insn): Correct already_warned
	logic.
	* testsuite/gas/arm/pr18347.s: Modify to generate duplicate
	warning without this patch.