gas: use common code for object attribute v1 & v2 parsing

gas: use common code for object attribute v1 & v2 parsing

Since the previous patch added all the code to be able to parse both
OAv1 and OAv2 directives, this patch switches OAv1 to use this common
code.
Additionally to the common code in obj-elf.c, the following backends
using a custom object attribute directive were impacted.
  - ARC
  - Arm
  - m68k
  - PowerPC
  - RISC-V
  - TI C6X
A parsing test for Arm had to be adapted to the error messages of the
new parser.

The gas and ld test suites were successfully run for the following
backends: S390, ARC, Arm, CSky, m68k, msp430, PowerPC, TI C6X, RISC-V,
AArch64, MIPS, SPARC.