[AIX][TLS] Generate .extern and .ref references to __tls_get_addr for local-exec accesses.

Compiling with TLS variables requires -pthread, but if the user omits this
option, the compiler will not show any obvious indication during compilation
that -pthread is needed for programs using TLS variables. Instead, the user will
experience a segmentation fault when running programs with TLS variables in them
and without specifying -pthread.

This patch aims to generate .extern/.ref references to __tls_get_addr[DS] for
local-exec accesses, in order to trigger an error from the linker to indicate
that there is an undefined symbol to __tls_get_addr. Doing so will remind the
user to compile/link with -pthread.

Differential Revision: https://reviews.llvm.org/D151335