…ff (#7162)

This handles the case where WASM_BIGINT is enabled by default by emcc.
Binaryen's JS bindings do not currently work with bigint (see #7163)

We previously had a build option for enabling BigInt support when
compiling with Emscripten, but the test suite did not actually pass with
it enabled. The problem was that the binaryen.js glue code assumed that
C API functions that took i64 parameters would be transformed to take a
pair of i32 parameters instead, but this assumption was incorrect when
BigInt support was enabled.

Emscripten has now enabled BigInt by default, so update binaryen.js to
use BigInt as well. Fix the JS API glue code to pass i64s as BigInts
rather than pairs of numbers and fix the tests accordingly. Also fix
some other small problems with the tests that only show up in debug
builds.

Resolves #7163.