all: use baremetal build tag

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

src/os/file_other.go

@@ -1,4 +1,4 @@
-// +build avr cortexm tinygo.riscv wasm
+// +build baremetal wasm
 
 package os
 

src/os/file_unix.go

@@ -1,4 +1,4 @@
-// +build darwin linux,!avr,!cortexm,!tinygo.riscv
+// +build darwin linux,!baremetal
 
 package os