Remove duplicate copies of trivial getcontextx.c

[emaste]

Summary

Only i386 and amd64 provide a non-trivial __getcontextx(). Use a common trivial implementation in gen/ for other architectures, rather than copying the file to each MD subdirectory.

[emaste]

I am not sure if This file was changed only by adding or removing whitespace is Phabricator getting confused or an accident in my diff. For committing this would be done with svn mv.

[emaste]

Closed by commit rS277078 (authored by @emaste).