ARC: Conditionalise certain relocations as provided by TLS tools only

uClibc mainline supports NPTL which in turns depends on TLS support in
the tools (gcc/binutils), which is yet to be merged in dev branches.

However there is some non NPTL code in uClibc, added as part of NPTL
effort, which relies on certain relocations only provided by NPTL
binutils. As a result building the current upstream even for LT.old
breaks.

So conditionalize that code on tools - indirecrtly though as depending
on NPTL.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>