[ARC] Don't convert _DYNAMIC@ to _GLOBAL_OFFSET_TABLE_

Historically the arc abi demanded that a GOT[0] should be referencible as
[pc+_DYNAMIC@gotpc].  Hence we convert a _DYNAMIC@gotpc to a GOTPC reference to
_GLOBAL_OFFSET_TABLE_.

This is no longer the case and uClibc and upcomming GNU libc don't expect this
to happen.

gas/ChangeLog:

    Vineet Gupta  <vgupta@synopsys.com>
    Cupertino Miranda  <cmiranda@synopsys.com>

	* config/tc-arc.c (md_undefined_symbol): Changed.
	* config/tc-arc.h (DYNAMIC_STRUCT_NAME): Removed.