Clean mechanism for specifying entry point for native eapps

[dkohlbre]

Currently if you make a native eapp (don't link to libc) without an lds/etc the linker will just choose the first function in the file as the entry point (doesn't see a _start).

We need a simple standard way to specify the entry point at compilation time of the eapp. We don't want to have to coordinate with the host app.

[dkohlbre]

Best solution to this is probably:
Have the eapp library contain the entry point, have it always call main(). Same as libc would do.

We may need to de-conflict this with using libc in enclaves as well.

[Nisiyama-Suzune]

Alternatively we may change int main to void _start in native eapps.
We may also set a linker flag -e main to direct the entry point to main.

[dkohlbre]

void _start wrapped into an ENCLAVE_START macro or similar is almost certainly the right answer.
Lets us make it slightly smarter later on, and keeps it working for now.