all: use baremetal build tag

This simplifies adding more baremetal targets, like a GameBoy Advance,
or baremetal x86 for unikernels.

src/syscall/syscall_baremetal.go

@@ -1,4 +1,4 @@
-// +build avr cortexm
+// +build baremetal
 
 package syscall
 

src/syscall/tables_baremetal.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build avr cortexm
+// +build baremetal
 
 package syscall