all: remove support for LLVM 9

This LLVM version breaks CI and is now relatively rather old anyway, so
remove support for it.

This also reverts a workaround for LLVM 9, see a9568932b ("maixbit:
workaround to avoid medium code model").

targets/k210.json

@@ -1,5 +1,6 @@
 {
 	"inherits": ["riscv64"],
 	"features": ["+a", "+c", "+m", "+f", "+d"],
-	"build-tags": ["k210", "kendryte"]
+	"build-tags": ["k210", "kendryte"],
+	"code-model": "medium"
 }