gdb: Use correct feature in tdesc-regs for ARC

gdb: Use correct feature in tdesc-regs for ARC

tdesc-regs.exp test fails for ARC because the test is not
using the correct XML files as target description.  With
this change, the correct directory and files are used.

v2 (after Andrew's remark [1]):
- Update the feature file names again.  Test results now:

  Test run by shahab on Tue Jan 26 11:31:16 2021
  Target is arc-default-elf32
  Host   is x86_64-unknown-linux-gnu

		  === gdb tests ===

  Schedule of variations:
      arc-nsim

  Running target arc-nsim
  Running /src/gdb/testsuite/gdb.xml/tdesc-regs.exp ...
  PASS: gdb.xml/tdesc-regs.exp: set tdesc file single-reg.xml
  PASS: gdb.xml/tdesc-regs.exp: cd to directory holding xml
  PASS: gdb.xml/tdesc-regs.exp: set tdesc filename test-extra-regs.xml...
  PASS: gdb.xml/tdesc-regs.exp: ptype $extrareg
  PASS: gdb.xml/tdesc-regs.exp: ptype $uintreg
  PASS: gdb.xml/tdesc-regs.exp: ptype $vecreg
  PASS: gdb.xml/tdesc-regs.exp: ptype $unionreg
  PASS: gdb.xml/tdesc-regs.exp: ptype $unionreg.v4
  PASS: gdb.xml/tdesc-regs.exp: ptype $structreg
  PASS: gdb.xml/tdesc-regs.exp: ptype $structreg.v4
  PASS: gdb.xml/tdesc-regs.exp: ptype $bitfields
  PASS: gdb.xml/tdesc-regs.exp: ptype $flags
  PASS: gdb.xml/tdesc-regs.exp: ptype $mixed_flags
  PASS: gdb.xml/tdesc-regs.exp: maintenance print reggroups
  PASS: gdb.xml/tdesc-regs.exp: core-only.xml: set tdesc filename...
  PASS: gdb.xml/tdesc-regs.exp: core-only.xml: ptype $extrareg

		=== gdb Summary ===

  # of expected passes		16

[1]
https://sourceware.org/pipermail/gdb-patches/2021-January/175465.html

gdb/testsuite/ChangeLog:

	* gdb.xml/tdesc-regs.exp: Use correct core-regs for ARC.