cmd/link: use 64k segment alignment on linux/arm

Otherwise 64k pages don't map correctly.

Fixes #18408.

Change-Id: I85f56682531566d1ff5c655640cd58509514aee8
Reviewed-on: https://go-review.googlesource.com/34629
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/link/internal/arm/obj.go

@@ -104,7 +104,7 @@ func archinit(ctxt *ld.Link) {
 			*ld.FlagDataAddr = 0
 		}
 		if *ld.FlagRound == -1 {
-			*ld.FlagRound = 4096
+			*ld.FlagRound = 0x10000
 		}
 
 	case obj.Hnacl: