Add initial support for LoongArch64

[heiher]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Add initial support for LoongArch64 Linux to crash-context, crash-handler and sadness-generator.

Related Issues

None

[xry111]

and should we also fill the FP or SIMD context?

[xry111]

Suggested change

	"li.w $t0, 0", // mcontext_t.__flags
	"st.w $t0, $a0, 440",
	"li.w $t0, 0", // sctx_info.magic
	"st.w $t0, $a0, 448",
	"li.w $t0, 0", // sctx_info.size
	"st.w $t0, $a0, 452",
	"st.w $zero, $a0, 440", // mcontext_t.__flags
	"st.d $zero, $a0, 448", // sctx_info.magic and sctx_info.size

[heiher]

and should we also fill the FP or SIMD context?

I think it's better to do it that way. The fact that LoongArch's struct mcontext_t doesn't reserved space for extcontext also exposes a drawback here, it prevents LoongArch from using the common allocation logic. Also, since LSX and LASX are both optional, we need to probe runtime support for them. I'll improve that part as well. Thanks!