[builtins] Try to ensure single copy of emulated TLS state

Multiple copies of emulated TLS state means inconsistent results when
accessing the same thread-local variable from different shared objects
(https://github.com/android/ndk/issues/1551). Making `__emutls_get_address`
be a weak default visibility symbol should make the dynamic linker
ensure only a single copy gets used at runtime. This is best-effort, but
the more robust approach of putting emulated TLS into its own shared
object would (a) be a much bigger change, and (b) shared objects are
pretty heavyweight, and adding a new one to a space-constrained
environment isn't an easy sell. Given the expected rarity of direct
accesses to emulated TLS variables across different shared objects, the
best-effort approach should suffice.

Reviewed By: danalbert, rprichard

Differential Revision: https://reviews.llvm.org/D107127

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