aws-c-common: emit unwind information for RISC-V

[ius]

Motivation for this change

The test suite for aws-c-common fails on RISC-V due to libgcc backtrace() returning 0, which in turn triggers a segfault due to underflow (see awslabs/aws-c-common#873).

This is fairly easy to reproduce:

#include <execinfo.h>
#include <stdio.h>

int main(int argc, char **argv[])
{
	void *buf[128];
	int n;

	n = backtrace(buf, 128);
	printf("backtrace() = %d\n", n);

	return 0;
}

Compile:

# riscv64-unknown-linux-gnu-gcc test.c && ./a.out 
backtrace() = 0

Passing -fasynchronous-unwind-tables helps. Note gcc_s needs to be explicitly linked to avoid runtime dlopen() failure:

# riscv64-unknown-linux-gnu-gcc -fasynchronous-unwind-tables test.c -lgcc_s && ./a.out 
backtrace() = 3

Notes

This appears to be gcc default for AArch64: [AArch64] Turn on -fasynchronous-unwind-tables and -funwind-tables by default.
and on x86_64 it works out of the box as well, so an alternate approach would be to patch the toolchain instead. If this doesn't turn out to be a widespread issue, just patching up this package works for me.

Resolution

Pass -fasynchronous-unwind-tables in CFLAGS for RISC-V.

This fixes functionality which depends on backtrace() and enables
running the test suite without error.

Things done

• Built on platform(s)
  • x86_64-linux
  • riscv64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 22.05 Release Notes (or backporting 21.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.