[mono][llvm] Mark the gc poll function as used so llvm doesn't optimi…

[vargaz]

…ze it away causing an assert inside 'opt'.

[lateralusX]

Validate that this works on a test assembly currently hitting this issue.

[akoeplinger]

A bunch of the wasm tests fail with something that looks related:

[wasm test] [03:27:17] info: ./dotnet.js:5625: SyntaxError: Unexpected token '.'
[wasm test] [03:27:17] info: var _gc.safepoint_poll = Module["_gc.safepoint_poll"] = function() {
[wasm test] [03:27:17] info:        ^
[wasm test] [03:27:17] info: SyntaxError: Unexpected token '.'
[wasm test] [03:27:17] info:     at loadScript (main.js:383:24)
[wasm test] [03:27:17] info:     at loadDotnet (main.js:391:12)
[wasm test] [03:27:17] info:     at main.js:394:1
[wasm test] [03:27:17] info: 
[wasm test] [03:27:17] info: Process v8 exited with 0

[akoeplinger]

do we need to backport this?

[vargaz]

No, it only happens on some newer windows platforms.