GAS ARM: Warn if the user creates a symbol with the same name as an instruction.

GAS ARM: Warn if the user creates a symbol with the same name as an instruction.

	PR gas/18347
gas	* config/tc-arm.c (md_undefined_symbol): Issue a warning message
	(if enabled) when the user creates a symbol with the same name as
	an ARM instruction.
	(flag_warn_syms): New static variable.
	(arm_opts): Add mwarn-syms and mno-warn-syms.
	* doc/c-arm.texi (ARM Options): Document the -m[no-]warn-syms
	options.

tests	* gas/arm/pr18347.s: New file: Test case.
	* gas/arm/pr18347.l: New file: Expected assembler output.
	* gas/arm/pr18347.d: New file: Test driver.