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.