gdb: PR 33384 invalid disassembler option message

This is the gdb part of fixing PR33384, where it is noted that an
error in a disassembler option prints the rest of the comma separated
option string rather than just the option in error.

Removing FOR_EACH_DISASSEMBLER_OPTION seemed a good idea to me, as we
then expose the strchr there which is useful in zero terminating the
option, and in the case of arm-tdep.c, to replace strcspn.  Also, if
the option is zero terminated we don't need disassembler_options_cmp.

Alternatively, you could do similarly to arm-tdep.c in disasm.c by
changing the error message to use %.*s with a length found by strcspn.
I rejected that smaller patch on the grounds that it makes for churn
in message translation.  I also prefer to see code using the standard
string functions.

Regression tested on x86_64-linux.  Message behaviour tested on
powerpc64le-linux and arm-linux-eabi.

	* arm-tdep.c (show_disassembly_style_sfunc): Don't use
	FOR_EACH_DISASSEMBLER_OPTION.  Use strchr needed for loop
	control to size option len.
	* disasm.c (set_disassembler_options): Don't use
	FOR_EACH_DISASSEMBLER_OPTION.  Overwrite comma in options with
	a zero.  Replace disassembler_options_cmp with strcmp.