[ELF][MIPS] Do not create a hidden definition for __tls_get_addr on MIPS

On most architectures the linker is required to optimize away any
references to __tls_get_addr in case of static linking. As usual
a special case is MIPS - libc defines __tls_get_addr itself because
there are no TLS optimizations for this architecture.

llvm-svn: 280664