The target triples have to match mostly to be able to link LLVM modules.
Linking LLVM modules is already possible (the triples already match),
but testing becomes much easier when they match exactly.
For macOS, I picked "macosx10.12.0". That's an old and unsupported
version, but I had to pick _something_. Clang by default uses
"macos10.4.0", which is much older.
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).
