`std.debug` should stop depending on `getcontext()`

[rootbeer]

Zig Version

0.15.0-dev.451+bd230215f

Steps to Reproduce and Observed Behavior

I believe the only current use of getcontext in the Zig library is in std.debug. From what I can tell, this code doesn't need a full ucontext_t (which includes a signal mask, floating point state, and a shadow stack). The existing debug code just needs the basic CPU registers. Perhaps just the mcontext_t register state would be sufficient?

I think moving away from getcontext() would make it easier to get the stack walking code working on other platforms (e.g., Android and OpenBSD don't provide a getcontext() at all, nor generally do freestanding and other targets). This also might make it simpler to get new platforms to support this debug code since they would just need a bit of arch-specific assembly to capture the basic registers.

Finally, the getcontext() API is "deprecated" (See https://man7.org/linux/man-pages/man3/getcontext.3.html: "POSIX.1-2008 removes these functions, citing portability issues, and recommending that applications be rewritten to use POSIX threads instead.") and has some hairy portability issues (it's a struct with embedded pointers that need to be fixed up when copied and its size can vary across C library versions and kernel versions).

Expected Behavior

std.debug uses locally defined assembly to capture register values, removing Zig's dependency on the getcontext() API.

[alexrp]

cc #23494

[alexrp]

To be clear, I intended for #23494 to be an incremental step towards what this issue is proposing. So, if this is something you feel like tackling, feel free to skip over the #23494 step.