[ARC] Fix parsing dtpoff relocation expression.

The assembler accepts dtpoff complex relocation expression like
identifier@dtpoff + const. However, it doesn't accept an expression such
as identifier@dtpoff@base + const. This patch solves this issue, and adds
a number of tests.

ld/
2016-09-14  Claudiu Zissulescu  <claziss@synopsys.com>

	* testsuite/ld-arc/tls-dtpoff.dd: New file.
	* testsuite/ld-arc/tls-dtpoff.rd: Likewise.
	* testsuite/ld-arc/tls-dtpoff.s: Likewise.
	* testsuite/ld-arc/tls-relocs.ld: Likewise.
	* testsuite/ld-arc/arc.exp: Add new tdpoff test.

gas/
2016-09-14  Claudiu Zissulescu  <claziss@synopsys.com>

	* testsuite/gas/arc/tls-relocs2.d: New file.
	* testsuite/gas/arc/tls-relocs2.s: Likewise.
	* config/tc-arc.c (tokenize_arguments): Accept offsets when base
	is used.