[sanitizer] Allow Fuchsia to use getauxval

Summary:
Fuchsia has `getauxval` (https://fuchsia.googlesource.com/zircon/+/master/third_party/ulib/musl/include/sys/auxv.h,
https://fuchsia.googlesource.com/zircon/+/master/third_party/ulib/musl/src/misc/getauxval.c)
so set SANITIZER_USE_GETAUXVAL to 1 for this platform.

Reviewers: alekseyshl, flowerhack

Reviewed By: flowerhack

Subscribers: srhines, kubamracek, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D42315

llvm-svn: 323002