[ELF][MIPS] Extend test case to check LA25 stubs creation for STO_MIPS_PIC symbols. NFC.

The only way to get an object file with symbols marked by the STO_MIPS_PIC
flag is to link PIC and non-PIC object files and generate a relocatable
output using '-r' command line option. Now LLD is able to generate a relocatable
output but does not mark PIC symbols by the STO_MIPS_PIC flag. So I have
to use binary input mips-sto-pic.o generated by GNU BFD linker.

llvm-svn: 265310