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).

.circleci/config.yml

@@ -119,7 +119,7 @@ commands:
             - lib/wasi-libc/sysroot
       - run:
           name: "Test TinyGo"
-          command: go test -v -timeout=20m -tags=llvm<<parameters.llvm>> ./cgo ./compileopts ./compiler ./interp ./transform .
+          command: go test -v -timeout=20m -tags=llvm<<parameters.llvm>> ./builder ./cgo ./compileopts ./compiler ./interp ./transform .
           no_output_timeout: 20m
       - run: make gen-device -j4
       - run: make smoketest XTENSA=0