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/riscv.json

@@ -8,6 +8,7 @@
 	"libc": "picolibc",
 	"cflags": [
 		"-Werror",
+		"-mno-relax",
 		"-fno-exceptions", "-fno-unwind-tables",
 		"-ffunction-sections", "-fdata-sections"
 	],