[ffigen] Generate C wrappers for capturing errno

[dcharkes]

Two subsequent FFI calls are not atomic from an OS point of view.
This means that accessing GetLastError and errno are flaky.
With the work progressing on native assets, our current idea to address this is to wrap the existing C call:

• Support for capturing errno across calls sdk#38832 (comment)

We'd rather not have users write wrappers by hand. So maybe we can explore generating them with FFIgen.

Some example uses of the APIs can be found in this abandoned CL: https://dart-review.googlesource.com/c/sdk/+/240847

Possible way of configuring wrappers

functions:
  capture-errno:
    - '.*'
  capture-GetLastError:
    - '.*'

And we should probably generate a return type that is a tuple with the potential error and the normal return value.