targets: add CPU property everywhere

This is for consistency with Clang, which always adds a CPU flag even if
it's not specified in CFLAGS.

This commit also adds some tests to make sure the Clang target-cpu
matches the CPU property in the JSON files.

This does have an effect on the generated binaries. The effect is very
small though: on average just 0.2% increase in binary size, apparently
because Cortex-M3 and Cortex-M4 are compiled a bit differently. However,
when rebased on top of https://github.com/tinygo-org/tinygo/pull/2218
(minsize), the difference drops to -0.1% (a slight decrease on average).

targets/esp8266.json

@@ -5,9 +5,6 @@
 	"scheduler": "tasks",
 	"linker": "xtensa-esp32-elf-ld",
 	"default-stack-size": 2048,
-	"cflags": [
-		"-mcpu=esp8266"
-	],
 	"rtlib": "compiler-rt",
 	"libc": "picolibc",
 	"linkerscript": "targets/esp8266.ld",