RISC-V: Enable RVC on ".option arch, +zca" etc.

Since the 'Zca' extension is the new base of the compressed instructions,
this commit enables RVC *also* when the 'Zca' extension is enabled
via ".option arch" directive.

gas/ChangeLog:

	* config/tc-riscv.c (s_riscv_option): Enable RVC also when the
	'Zca' extension is enabled after an ".option arch" directive.