[arm] Parse tag names for eabi_attribute directive.

This commit adds the preliminary support to parse the tag name of the
eabi_attribute directive.  For example,

    .eabi_attribute Tag_ARM_ISA_use, 1

should be equivalent to:

    .eabi_attribute 8, 1

Most tag names are mapped and tested.  However, due to the incomplete
eabi_attribute directive parser, following tags are missing from the
test case: Tag_CPU_raw_name, Tag_CPU_name, Tag_compatibility,
Tag_also_compatible_with, and Tag_conformance.