RISC-V: PR32014, .option directives shuoldn't affect elf attribute.

The .option arch/rvc/norvc/push/pop directives can only take effect for a
small/large specific code region, so they are not file-level architecture
setting.  They should only affect the mapping symbols only rather than the
file-level elf architecture attribute.  Otherwise, the elf architecture
attribute will appear to missing some extensions when -flto merges files
with different .option architecture settings.

gas/
	PR 32014
	* config/tc-riscv.c (file_arch_str): New const char *, rather than the
	arch_str in the riscv_rps_as.subset_list, it's file-level so only be
	affected by .attribute arch directive.
	(riscv_reset_subsets_list_arch_str): Renamed to riscv_set_arch_str, and
	also can handle both file_arch_str and arch_str in subset_list, just
	give the pointer address as the input.
	(riscv_set_arch): Called by -march and .attribute arch, so set both
	file_arch_str and arch_str in subset_list.
	(s_riscv_option): Updated .option arch/rvc/norvc/push/pop that only
	set the arch_str in subset_list.
	(riscv_write_out_attrs): Output elf architecture attribute according to
	file_arch_str.  Freed file_arch_str.
	* doc/c-riscv.texi: Added destrbution that .option directives shouldn't
	affect the elf attribute settings.
	* testsuite/gas/riscv/option-arch.s: From option-arch-01/02/03 merged.
	* testsuite/gas/riscv/option-arch-dis.d: Likewise, for dis-assembler.
	* testsuite/gas/riscv/option-arch-attr.d: Likewise, to check readelf -A.