[compiler-rt][sanitizer] Have all OOM-related error messages start with the same format

This way downstream tools that read sanitizer output can differentiate between OOM errors
reported by sanitizers from other sanitizer errors.

Changes:

- Introduce ErrorIsOOM for checking if a platform-specific error code from an "mmap" is an OOM err.
- Add ReportOOMError which just prepends this error message to the start of a Report call.
- Replace some Reports for OOMs with calls to ReportOOMError.
- Update necessary tests.

Differential Revision: https://reviews.llvm.org/D127161