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/syscall/syscall_darwin.go

@@ -12,7 +12,7 @@ package syscall
 //         return errno;
 //     }
 //
-//go:export __error
+//export __error
 func libc___error() *int32
 
 // getErrno returns the current C errno. It may not have been caused by the last

src/syscall/syscall_libc.go

@@ -53,5 +53,5 @@ func splitSlice(p []byte) (buf *byte, len uintptr) {
 }
 
 // ssize_t write(int fd, const void *buf, size_t count)
-//go:export write
+//export write
 func libc_write(fd int32, buf *byte, count uint) int