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/fe310.json

@@ -1,5 +1,6 @@
 {
 	"inherits": ["riscv32"],
+	"cpu": "sifive-e31",
 	"features": ["+a", "+c", "+m"],
 	"build-tags": ["fe310", "sifive"]
 }