all: use baremetal build tag

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

src/runtime/arch_arm.go

@@ -1,4 +1,4 @@
-// +build arm,!avr,!cortexm,!tinygo.riscv
+// +build arm,!baremetal
 
 package runtime
 

src/runtime/baremetal.go

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

src/runtime/gc_globals_conservative.go

@@ -1,5 +1,5 @@
 // +build gc.conservative
-// +build cortexm tinygo.riscv
+// +build baremetal
 
 package runtime
 

src/runtime/gc_globals_precise.go

@@ -1,5 +1,5 @@
 // +build gc.conservative
-// +build !cortexm,!tinygo.riscv
+// +build !baremetal
 
 package runtime
 

src/runtime/gc_stack_portable.go

@@ -1,5 +1,5 @@
 // +build gc.conservative
-// +build !cortexm,!tinygo.riscv
+// +build !baremetal
 
 package runtime
 

src/runtime/gc_stack_raw.go

@@ -1,5 +1,5 @@
 // +build gc.conservative
-// +build cortexm tinygo.riscv
+// +build baremetal
 
 package runtime
 

src/runtime/runtime_unix.go

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