[libc++] Fix incorrect main() signatures in the tests

Those creep up from time to time. We need to use `int main(int, char**)`
because in freestanding mode, `main` doesn't get special treatment and
special mangling, so we setup a symbol alias from the mangled version of
`main(int, char**)` to `extern "C" main`. That only works if all the tests
are consistent about how they define their main function.