Implement @mod and fix bugs with divFloor for wasm

[riverbl]

Implement lowering code for @mod on integers in the stage2 wasm backend (@mod on floats remains unimplemented).

Fix invalid wasm being produced for @divFloor on signed integers by the stage2 wasm backend. wasm comparison instructions (e.g., less than) always output an i32, and select takes an i32 as the condition, but the previous @divFloor implementation assumed that when comparing or selecting between i64, the output / condition would be an i64.

This pull request also renames signAbsValue in wasm/CodeGen.zig to signExtendInt, as I found the previous name misleading.

The new @divFloor implementation 0 extends its result when used on type iN where N < 64, N != 32 - see #16807 (comment).

[Luukdegram]

Thank you for your contribution. I'd like to see a behavior test enabled that triggers this instruction as part of this PR before accepting. The rest is just some minor aesthetic changes, which aren't necessarily blocking this PR from being merged.

[riverbl]

I've added a test (copied from behavior/int_div.zig, plus some float test cases) covering @divFloor and @mod. It's only enabled for the stage2 wasm backend, and has a TODO comment advising that it be deleted once the division tests in behavior/int_div.zig and behavior/math.zig pass with the stage2 wasm backend.

[Luukdegram]

Thank you, great work!