ci: don't include prebuilt libraries in the release

These libraries will be automatically built when needed and cached.

The main reason these were needed is for play.tinygo.org, but I've now
prebuilt them there directly (so they don't need to be built for every
tarball).

builder/builtins.go

@@ -169,12 +169,12 @@ var avrBuiltins = []string{
 	"avr/udivmodqi4.S",
 }
 
-// CompilerRT is a library with symbols required by programs compiled with LLVM.
-// These symbols are for operations that cannot be emitted with a single
+// libCompilerRT is a library with symbols required by programs compiled with
+// LLVM. These symbols are for operations that cannot be emitted with a single
 // instruction or a short sequence of instructions for that target.
 //
 // For more information, see: https://compiler-rt.llvm.org/
-var CompilerRT = Library{
+var libCompilerRT = Library{
 	name: "compiler-rt",
 	cflags: func(target, headerPath string) []string {
 		return []string{"-Werror", "-Wall", "-std=c11", "-nostdlibinc"}