MIPS/opcodes: Add TX39 CP0 register names

The TX39 core has its distinct set of CP0 registers[1], so it needs a
separate table to hold their names.  Add a test case accordingly.

References:

[1] "32-Bit RISC Microprocessor TX39 Family Core Architecture User's
    Manual", Toshiba, Jul. 27, 1995, Section 2.2.2 "System control
    coprocessor (CP0) registers", pp. 9-10

	opcodes/
	* mips-dis.c (mips_cp0_names_r3900): New variable.
	(mips_arch_choices): Use it rather than `mips_cp0_names_numeric'
	for "r3900".

	gas/
	* testsuite/gas/mips/cp0-names-r3900.d: New test.
	* testsuite/gas/mips/mips.exp: Run it.