wasm32: div by a run-time zero traps instead of raising

uses SysUtils;
b := 0;
try WriteLn(a div b);
except on E: EDivByZero do WriteLn('caught ', E.ClassName); end;
{ FPC, x86-64, i386, arm32, aarch64, riscv32, xtensa: caught EDivByZero }
{ wasm32: wasm trap: integer divide by zero, exit 134 }

Cause and fix

wasm's div/rem instructions trap when the divisor is zero. The native backends first call the handler that DivZeroCheckProc names. That handler is PXXDivZero, which raises EDivByZero through SysUtils' hook and otherwise stops with RE 200. WasmIntBinop emitted the instruction with no check.

WasmDivZeroGuard now runs before each integer div/mod. It stores the divisor in a scratch local, and if the divisor is zero it calls the handler and sets the divisor to 1. The instruction therefore cannot trap before WasmEmitExcCheck unwinds, the same way it does after any other call. When DivZeroCheckProc is -1 (--no-div-check, or the ESP platforms) nothing is emitted, as on the other targets.

Measured (2026-09-29, fixedpoint 4de42d8ec324)

• test/test_div_by_a_run_time_zero_raises_on_wasm32.pas covers:

  • div and mod on Integer, Int64, Cardinal and QWord;
  • a divide inside a call argument;
  • a divide on the right of a short-circuit and;
  • non-zero divisors, including a negative one.

  It equals FPC 3.2.2 on x86-64, i386, arm32, aarch64, riscv32, xtensa windowed and wasm32. With the previous compiler, only wasm32 differs (it stops at the first case, exit 134). The rows are x86-64, i386 and wasm32. The wasm32 row fails with the previous compiler.

• Raised in a nested function, the exception propagates through two frames (one of them holds a managed AnsiString local) to the caller's handler, the same as on x86-64.

• Without SysUtils, wasm32 now prints Runtime error 200 (division by zero) as x86-64 does. The process still does not exit 200 under wasmtime, because WASI refuses any exit status of 126 or higher. A plain Halt(200) does the same, so that limit comes from the runner and predates this fix.

• The 79 wasm32 Makefile blocks: no block changes status.