[scudo] Skip building scudo standalone if sys/auxv.h can't be found

Summary:
Since commit c299d198 scudo
standalone can't be built without including sys/auxv.h.
I do not have that file on my system, and my builds have failed
when trying to simply build "all" runtimes. Assuming that "all"
means "all possible given the current environment" we need to
guard the setting of COMPILER_RT_HAS_SCUDO_STANDALONE=TRUE by
first checking if sys/auxv.h can be found.

Reviewers: pcc, cryptoad

Reviewed By: pcc

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73631