runtime: always build panic32.go

Avoids problems with arm64 ILP32 mode.  We might want to handle that
mode better in general, but always building panic32.go is simple and
fixes the build.

Change-Id: I57c0d2ba51176be36ab8020e60ee62d4aa397e28
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192723
Reviewed-by: Cherry Zhang <cherryyz@google.com>
diff --git a/libgo/go/runtime/panic32.go b/libgo/go/runtime/panic32.go
index f68d4c7..c8861c9 100644
--- a/libgo/go/runtime/panic32.go
+++ b/libgo/go/runtime/panic32.go
@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build 386 amd64p32 arm mips mipsle m68k nios2 sh shbe
-
 package runtime
 
 import _ "unsafe" // for go:linkname