Enable the constraints checking for assembler by default if possible.

[Nelson1225]

I get the request recently that the assembler's constraints checking break hardware exception test cases. Therefore I have disabled the checking in assembler by default, and people don't get the constraints error when writing assembly code. But we should enable the checking for the code which the compiler emits.

Beside, GCC also checks whether the assembler supports the options used to enable and disable the constraints checking. The basic logic is shown as follows,

	GAS support constraints options	GAS doesn't support constraints options
GCC default	Output .option checkconstraints	Don't output the .option checkconstraints
GCC -mcheck-constraints	Output .option checkconstraints	Report error message that GAS doesn't support .option checkconstraints
GCC -mno-check-constraints	Don't output the.option checkconstraints	Don't output the.option checkconstraints

One more thing is that, the gcc_GAS_CHECK_FEATURE doesn't seem to work for checking the .option xxx. I think the reason may be that GAS report warning message rather than error when the .option xxx isn't supported. However, I have added both GAS options and .option directives for the
constraints checking, so I check them both in the gcc_GAS_CHECK_FEATURE.

gcc/
* config.in: Regenerated.
* configure: Likewise.
* configure.ac: Check whether GAS support constraints options.
* config/riscv/riscv.c (riscv_option_override): If user don't set the
`m[no-]check-constraints` option and GAS support the constraints
checking, then GCC should enable it by default. If user set the
`mcheck-constraints` options but the checking isn't supported in GAS,
then we should report error message.
(riscv_file_start): Emit `.option checkconstraints` if needed.
* config/riscv/riscv.opt (mcheck-constraints): New option.

gcc/testsuite/
* gcc.target/riscv/constraints-1.c: New.
* gcc.target/riscv/constraints-2.c: Likewise.

[Nelson1225]

The related support in binutils riscvarchive/riscv-binutils-gdb#193

[jim-wilson]

You can pass --fatal-warnings to GNU as to change warnings into errors. That might fix your configure patch problem.

[Nelson1225]

You can pass --fatal-warnings to GNU as to change warnings into errors. That might fix your configure patch problem.

Thanks this works :)

[kito-cheng]

Verified on my side, I gonna merge this.

[Nelson1225]

Thanks for helps :)