all: use -Qunused-arguments only for assembly files

The -Qunused-arguments flag disables the warning where some flags are
not relevant to a compilation. This commonly happens when compiling
assembly files (.s or .S files) because some flags are specific to C and
not relevant to assembly.
Because practically all baremetal targets need some form of assembly,
this flag is added to most CFlags. This creates a lot of noise. And it
is also added for compiling C code where it might hide bugs (by hiding
the fact a flag is actually unused).

This commit adds the flag to all assembly compilations and removes them
from all target JSON files.

targets/stm32f4disco.json

@@ -1,9 +1,6 @@
 {
   "inherits": ["cortex-m4"],
   "build-tags": ["stm32f4disco", "stm32f407", "stm32f4", "stm32"],
-  "cflags": [
-    "-Qunused-arguments"
-  ],
   "linkerscript": "targets/stm32f407.ld",
   "extra-files": [
     "src/device/stm32/stm32f407.s"