all: add target-features string to all targets

This makes sure that the LLVM target features match the one generated by
Clang:

  - This fixes a bug introduced when setting the target CPU for all
    targets: Cortex-M4 would now start using floating point operations
    while they were disabled in C.
  - This will make it possible in the future to inline C functions in Go
    and vice versa. This will need some more work though.

There is a code size impact. Cortex-M4 targets are increased slightly in
binary size while Cortex-M0 targets tend to be reduced a little bit.
Other than that, there is little impact.

targets/k210.json

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