stage2 tests: improve C backend testing

Add -Werror -pedantic -std=c89 to make sure our outputted C code is
squeaky clean.

Allow garbage to be printed to stderr for Execution tests because
wasmtime is printing a bogus warning, causing the CI to fail.
3d79ae2be3Andrew Kelley committed on 12/30/2020, 3:01:18 AMparentd18b678
1 file changedLine totals unavailable