[SCUDO] add cmake options for custom sysroot

These options will allow the SCUDO standalone to be built with custom
headers. Specifically, this patch will enable building with the
LLVM-libc headers.

Reviewed By: abrachet

Differential Revision: https://reviews.llvm.org/D135702

GitOrigin-RevId: b9663ebbf825eed3fc45fbb7d4773271ee2ca259
2 files changed