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/maixbit.ld

@@ -1,11 +1,7 @@
 
 MEMORY
 {
-    /* This is a software workaround to avoid using the medany code model
-       which causes the llvm9 build to fail. 0x80000000 is mirorred at
-       0xffffffff80000000. This is not needed anymore on llvm10.
-       https://github.com/rust-embedded/riscv-rt/issues/25 */
-    RAM (xrw) : ORIGIN = 0xffffffff80000000, LENGTH = 6M
+    RAM (xrw) : ORIGIN = 0x80000000, LENGTH = 6M
 }
 
 _stack_size = 2K;