Make up for the lack of intrinsic headers in Open Enclave

[achamayou]

Fix the main remaining problem that prevents building oesnmalloc with -nostdinc. There's still a issue with the order in which headers are copied in the OE build but that's nothing to do with snmalloc.

Paging @anakrish because I can't work how to add him to Reviewers for some reason.

It is probably worth noting that this can be removed if/when
a subset of openenclave/openenclave#1766 is implemented in OE. That's been open for over a year though, so I would still like to have this change considered.

[mjp41]

LGTM

[achamayou]

I'm not quite sure how this happened or why:

##[error]Bash exited with code '128'.
##[error]Bash wrote one or more lines to the standard error stream.
##[error]fatal: bad object 6b4c75a659ce6f15ef32a2ec7661af57aceb3664

[achamayou]

/azp run

[azure-pipelines]

No pipelines are associated with this pull request.

[anakrish]

Thanks for doing this @achamayou