FFI implementation libc getuid on Linux

[pcwizz]

I relatively often find code I want to run with Miri that depends on libc getuid when targeting linux. At least to my mind it is a basic operating system API worthy of a miri implementation that would enable more miri code coverage.

[RalfJung]

We in fact already support getuid, but only when called from inside std... should be easy to change that to allow all code to call it.

The only hard part is a policy decision: currently it always returns 1000. Should we make it return the real ID instead? But what would that even mean when we run on a Windows host?

[pcwizz]

Awesome! I think for my purposes 1000 is sufficient. No need to over complicate it.