xtensa: gas: support optional immediate simcall parameter

xtensa: gas: support optional immediate simcall parameter

Starting with RH.0 release Xtensa ISA adds immediate parameter to
simcall opcode. For assembly source compatibility treat "simcall"
instruction without parameter as "simcall 0" when parameter is required.

2020-04-29  Max Filippov  <jcmvbkbc@gmail.com>
gas/
	* config/tc-xtensa.c (XTENSA_MARCH_EARLIEST): Define macro as 0
	if it's not defined.
	(microarch_earliest): New static variable.
	(xg_translate_idioms): Translate "simcall" to "simcall 0" when
	simcall opcode has mandatory parameter.
	(xg_init_global_config): Initialize microarch_earliest.