[ELF] Add a better test for the multi-CU .gdb_index bug that D54361 fixed

gdb-index-multiple-cu-2.s puts the symbol in question to another object file %t1.o, so that its CuIndex is affected by the number of CUs in %t.o

Also change `Kind:` in a comment to `Attributes:` as a follow-up of D54480 and D54481

llvm-svn: 346796