all: change //go:export to //export

This is the kind that is used in Go (actually CGo) for exporting
functions. I think it's best to use //export instead of our custom
//go:export pragma, for consistency (they are equivalent in TinyGo).
Therefore I've updated all instances to the standard format (except for
two that are updated in https://github.com/tinygo-org/tinygo/pull/1024).

No smoke tests changed (when comparing the output hash), except for some
wasm tests that include DWARF debug info and tend to be flaky anyway.

src/runtime/arch_wasm.go

@@ -14,7 +14,7 @@ const TargetBits = 32
 //go:extern __heap_base
 var heapStartSymbol [0]byte
 
-//go:export llvm.wasm.memory.size.i32
+//export llvm.wasm.memory.size.i32
 func wasm_memory_size(index int32) int32
 
 var (