riscv64: subtract zero from signed integers

[ivanstepanovftw]

Seems like riscv64 does not know how to subtract zero from signed integers.

Originally posted by @ivanstepanovftw in #24317 (comment)

[ivanstepanovftw]

By adding more behavior tests to cover SPIR-V codegen bug described in #24281:

+    try testSubWithOverflow(u8, 42, 0, 42, 0);
+    try testSubWithOverflow(i8, 42, 0, 42, 0);
+    try testSubWithOverflow(i8, -42, 0, -42, 0);

, I noticed failed test for riscv64:
https://github.com/ziglang/zig/actions/runs/16032188851/job/45235237633

test
+- test-modules
   +- test-behavior
      +- run test behavior-riscv64-linux-none-baseline_rv64-Debug-selfhosted-no-lld
         +- compile test Debug riscv64-linux-none 1 errors
/home/ci/actions-runner6/_work/zig/zig/test/behavior/math.zig:1114:1: error: TODO: airSubWithOverflow signed 8
fn testSubWithOverflow(comptime T: type, a: T, b: T, sub: T, bit: u1) !void {
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: the following command failed with 1 compilation errors:
/home/ci/actions-runner6/_work/zig/zig/build-debug-llvm/stage3-debug/bin/zig test -fno-llvm -fno-lld -ODebug -target riscv64-linux-none -mcpu baseline+v+zbb -Mroot=/home/ci/actions-runner6/_work/zig/zig/test/behavior.zig --cache-dir /home/ci/actions-runner6/_work/zig/zig/zig-local-cache --global-cache-dir /home/ci/actions-runner6/_work/zig/zig/zig-global-cache --name test -L /home/ci/deps/zig+llvm+lld+clang-x86_64-linux-musl-0.15.0-dev.233+7c85dc460/lib -I /home/ci/deps/zig+llvm+lld+clang-x86_64-linux-musl-0.15.0-dev.233+7c85dc460/include --zig-lib-dir /home/ci/actions-runner6/_work/zig/zig/lib --listen=-