loongarch: add back support for object attributes v1

A previous commit [1] mistakenly removed support for Object Attributes
for LoongArch targets.

This patch adds back the OA feature into Gas for the LoongArch targets
following the new approach which consists in:
- defining TC_OBJ_ATTR to 1 in tc-loongarch.h
- enabling the compilation of obj-elf-attr.c in gas/configure.ac

[1]: 28a51085ae59fdb530fbdce1e2a7d160667e3007