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/cortex-m4.json

@@ -3,7 +3,6 @@
 	"llvm-target": "armv7em-none-eabi",
 	"cflags": [
 		"--target=armv7em-none-eabi",
-		"-mfloat-abi=soft",
-		"-Qunused-arguments"
+		"-mfloat-abi=soft"
 	]
 }