[scudo] Bump the Android API level requirement to 21 for getauxval

Summary:
`getauxval` was introduced in 18 & 21 depending on the architecture. Bump the
requirement to 21.

It also turns out that the NDK is finicky: NDK r13b doesn't include sys/auxv.h
when creating a standalone toolchain at API level 19 for ARM. So 18 didn't work
well with older NDKs.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: aemerson, srhines, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D39905

llvm-svn: 317907