MIPS/opcodes: Only call `bfd_mips_elf_get_abiflags' if BFD64

MIPS/opcodes: Only call `bfd_mips_elf_get_abiflags' if BFD64

Complement commit 5e7fc731f80e ("MIPS/opcodes: Also set disassembler's
ASE flags from ELF structures") and fix an `--enable-targets=all' GDB
build regression on 32-bit hosts where the MIPS target is a secondary:

../opcodes/libopcodes.a(mips-dis.o): In function `set_default_mips_dis_options':
mips-dis.c:(.text+0x906): undefined reference to `bfd_mips_elf_get_abiflags'
collect2: error: ld returned 1 exit status
make[2]: *** [gdb] Error 1

by avoiding making a call to the `bfd_mips_elf_get_abiflags' function,
which is not available, because there is no MIPS/ELF BFD included in
32-bit BFD builds.  This call is only made from a conditional code block
guarded by a check against `bfd_target_elf_flavour', which is dead in
such a configuration, however cannot be optimized away by the compiler.
Also some other MIPS BFDs may be available, such as a.out, ECOFF or PE,
so the disassembler has to remain functional.

	opcodes/
	* mips-dis.c (set_default_mips_dis_options) [BFD64]: Only call
	`bfd_mips_elf_get_abiflags' here.