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]: 28a51085ae

gas/config/tc-loongarch.h

@@ -149,4 +149,7 @@ struct reloc_info
 #define md_finish loongarch_md_finish
 extern void loongarch_md_finish (void);
 
+/* The target supports Object Attributes.  */
+#define TC_OBJ_ATTR 1
+
 #endif

gas/configure

@@ -12701,7 +12701,7 @@ _ACEOF
 
     # Does the target support Object Attributes ?
     case ${cpu_type} in
-      arc* | arm* | csky | m68k | mips* | msp430 | ppc* \
+      arc* | arm* | csky | loongarch | m68k | mips* | msp430 | ppc* \
       | riscv* | s390* | sparc* | tic6x)
 	for f in config/obj-elf-attr.o; do
 	  case " $extra_objects " in

gas/configure.ac

@@ -434,7 +434,7 @@ changequote([,])dnl
 
     # Does the target support Object Attributes ?
     case ${cpu_type} in
-      arc* | arm* | csky | m68k | mips* | msp430 | ppc* \
+      arc* | arm* | csky | loongarch | m68k | mips* | msp430 | ppc* \
       | riscv* | s390* | sparc* | tic6x)
 	for f in config/obj-elf-attr.o; do
 	  case " $extra_objects " in