compileopts: fix race condition

Appending to a slice can lead to a race condition if the capacity of the
slice is larger than the length (and therefore the returned slice will
overwrite some fields in the underlying array).

This fixes that race condition. I don't know how severe this particular
one is. It shouldn't be that severe, but it is a bug and it makes it
easier to hunt for possibly more serious race conditions.

compileopts/config.go

@@ -74,7 +74,8 @@ func (c *Config) GOARM() string {
 
 // BuildTags returns the complete list of build tags used during this build.
 func (c *Config) BuildTags() []string {
-	tags := append(c.Target.BuildTags, []string{
+	tags := append([]string(nil), c.Target.BuildTags...) // copy slice (avoid a race)
+	tags = append(tags, []string{
 		"tinygo",                                     // that's the compiler
 		"purego",                                     // to get various crypto packages to work
 		"math_big_pure_go",                           // to get math/big to work