Implement `prctl` as specified in
https://man7.org/linux/man-pages/man2/prctl.2.html.
This patch also includes test cases covering two simple use cases:
1. `PR_GET_NAME/PR_SET_NAME`: where userspace data is passed via arg2.
2. `PR_GET_THP_DISABLE`: where return value is passed via syscal retval.
These headers are uncommonly used, and from extensions, but some basic
support is needed. Macros have been added where available.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D135731