Implement emulated support for getpid.

[sunfishcode]

Add a simple getpid emulation function which just returns a fixed
value, for applications that just use it for logging.

[sbc100]

I won't if this fine grained approach to emulation is scale-able.

One alternative would be have a single -lwasi-compat and -DWASI_COMPAT ?

this change lgtm though

[sunfishcode]

It kind of feels right to keep them separate right now, because they imply different kinds of emulation. getpid is just "here's a bogus value". mmap is "i'll actually read the file for you, but in a different way". And so on. But yeah, I could see a "enable all" mode becoming useful.