tcg/mips: Create and use TCG_REG_TB

tcg/mips: Create and use TCG_REG_TB

This vastly reduces the size of code generated for 64-bit addresses.
The code for exit_tb, for instance, where we load a (tagged) pointer
to the current TB, goes from

0x400aa9725c:  li       v0,64
0x400aa97260:  dsll     v0,v0,0x10
0x400aa97264:  ori      v0,v0,0xaa9
0x400aa97268:  dsll     v0,v0,0x10
0x400aa9726c:  j        0x400aa9703c
0x400aa97270:  ori      v0,v0,0x7083

to

0x400aa97240:  j        0x400aa97040
0x400aa97244:  daddiu   v0,s6,-189

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>