Revert "[dev.link] cmd/link: remove buffered file I/O from OutBuf"

This reverts commit b2def42d9e.

Reason for revert: trybots failing

Change-Id: I920be6d8de158b1e513154ac0eb0c8fa0cffa9f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/228657
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/link/internal/wasm/asm.go

@@ -187,6 +187,8 @@ func asmb2(ctxt *ld.Link) {
 	if !*ld.FlagS {
 		writeNameSec(ctxt, len(hostImports), fns)
 	}
+
+	ctxt.Out.Flush()
 }
 
 func lookupType(sig *wasmFuncType, types *[]*wasmFuncType) uint32 {