s390: gas: Support vector register names in CFI directives

Convert vector register names v0-v31 to DWARF register numbers when used
in CFI directives.  Convert a vector register number (0..31) to its
respective DWARF register number (16..31, 68..83) as specified in the
s390 64-bit (s390x) ELF ABI [1] as follows:
Right rotate the least significant three bits and add 16 for register
numbers 0..15 or 68 for register numbers 16..31.

Add a test case for the use of vector register (VR) names in CFI
directives.

[1]: s390x ELF ABI, https://github.com/IBM/s390x-abi/releases

gas/
	* config/tc-s390.c (tc_s390_regname_to_dw2regnum): Convert
	vector register names (v0..v31) to DWARF register numbers.

gas/testsuite/
	* gas/s390/cfi-regnames.d: Add test for VR names in in CFI
	directives.
	* gas/s390/cfi-regnames.s: Likewise.
	* gas/s390/cfi-regnames-err.l: Likewise.
	* gas/s390/cfi-regnames-err.s: Likewise.

Signed-off-by: Jens Remus <jremus@linux.ibm.com>