MIPS/opcodes: Only examine ELF file structures if SYMTAB_AVAILABLE

MIPS/opcodes: Only examine ELF file structures if SYMTAB_AVAILABLE

Correct commit 640c0ccdc980 ("some objdump -M options, better reg
dumps"), <https://sourceware.org/ml/binutils/2002-12/msg00706.html>, and
only execute code setting up disassembler options based on ELF file
structures if SYMTAB_AVAILABLE is set.

	opcodes/
	* mips-dis.c (set_default_mips_dis_options) [SYMTAB_AVAILABLE]:
	Only examine ELF file structures here.