emscripten_get_now should have no leading underscore

[hoodmane]

According to @sbc100 in https://github.com/emscripten-core/emscripten/issues/17106#issuecomment-1143836563 `emscripten_core:

It looks like that side module is trying to import this symbol with an _ prefix. But that symbol doesn't start with an underscore:

https://github.com/emscripten-core/emscripten/blob/922cf56100de7dd162593f64481c73d16931643e/system/include/emscripten/emscripten.h#L86

See https://github.com/emscripten-core/emscripten/issues/17106

I have confirmed that this fix works on https://github.com/pyodide/pyodide/pull/2378 though it requires an additional patch to Emscripten to add a signature for emscripten_get_now (see linked Emscripten issue). This additional Emscripten patch is only needed to use instant in a dynamically linked side module, if it's in the main module it is unnecessary.

[hoodmane]

Fixes #35.