[scudo] getauxval alternative for Android

Summary:
`getauxval` was introduced with API level 18. In order to get things to work
at lower API levels (for the toolchain itself which is built at 14 for 32-bit),
we introduce an alternative implementation reading directly from
`/proc/self/auxv`.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: srhines, llvm-commits

Differential Revision: https://reviews.llvm.org/D37488

llvm-svn: 312653