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/atmega2560.json

@@ -3,9 +3,6 @@
     "cpu": "atmega2560",
     "build-tags": ["atmega2560", "atmega"],
     "serial": "uart",
-    "cflags": [
-        "-mmcu=atmega2560"
-    ],
     "ldflags": [
         "-mmcu=avr6",
         "-Wl,--defsym=_stack_size=512"