[libunwind] Don't use dl_iterate_phdr if __ANDROID_API__ < 21

Summary:
On ARM, dl_iterate_phdr is only implemented in the Android NDK version 21 or later:
https://android.googlesource.com/platform/prebuilts/ndk/+/dev/platform/sysroot/usr/include/link.h#55

Reviewers: thakis, danalbert

Reviewed By: danalbert

Subscribers: dtzWill, aemerson, srhines, kristof.beyls

Differential Revision: https://reviews.llvm.org/D39468

llvm-svn: 317125