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.
    
    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/192723

From-SVN: r275237

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-11fd9208f8545e882f945d3ed86fcc33abf1a61b
+2444eb1e8c697531f8beb403679e4ab00b16dbf5
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

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