[Android] Use ELF TLS for Android API level 29+

Use ELF TLS by default instead of emulated TLS for Android build targets when
the API level is >= 29.

Android Q features full TLS support. See release notes
(https://developer.android.com/preview/features#elf-tls) and implementation details
(https://android.googlesource.com/platform/bionic/+/master/docs/elf-tls.md#Workaround_Use-Variant-2-on-arm32_arm64).

Reviewed By: rprichard, danalbert

Differential Revision: https://reviews.llvm.org/D147849