opcodes/csky: return the default disassembler when there is no bfd

opcodes/csky: return the default disassembler when there is no bfd

The disassembler function should return a valid disassembler function
even when there is no BFD present.  This is implied (I believe) by the
comment in dis-asm.h which says the BFD may be NULL.  Further, it
makes sense when considering that the disassembler is used in GDB, and
GDB may connect to a target and perform debugging even without a BFD
being supplied.

This commit makes the csky_get_disassembler function return the
default disassembler configuration when no bfd is supplied, this is
the same default configuration as is used when a BFD is supplied, but
the BFD has no attributes section.

Before the change configuring GDB with --enable-targets=all and
running the tests gdb.base/all-architectures-2.exp results in many
errors, but after this change there are no failures.

opcodes/ChangeLog:

	* csky-dis.c (csky_get_disassembler): Don't return NULL when there
	is no BFD.