Install final length of TLB refill handler rather than 256 bytes

[frno7]

The R5900 TLB refill handler is limited to 128 bytes, corresponding to 32 instructions. Commit eaccf2f32090df98f70b47edf07d15a82792e428 is a provisional fix. Commit 8ff93271c3507155222567cc66e137234d419e78 verifies that the R5900 TLB refill handler does not overflow.

Installing a 256 byte TLB refill handler for the R5900 at address 0x80000000 overwrites the performance counter (see #26) handler at address 0x80000080, according to the TX79 manual, p. 5-7:

Table 5-2. Exception Vectors for Level 1 exceptions

Exceptions	Vector Address
	BEV = 0	BEV = 1
TLB Refill (EXL = 0)	0x8000 0000	0xBFC0 0200
TLB Refill (EXL = 1)	0x8000 0180	0xBFC0 0380
Interrupt	0x8000 0200	0xBFC0 0400
Others	0x8000 0180	0xBFC0 0380

Table 5-3. Exception Vectors for Level 2 exceptions

Exceptions	Vector Address
	DEV = 0	DEV = 1
Reset, NMI	0xBFC0 0000	0xBFC0 0000
Performance Counter	0x8000 0080	0xBFC0 0280
Debug, SIO	0x8000 0100	0xBFC0 0300

[frno7]

A patch was posted to the mailing list for Linux on MIPS. The proposal is to fold it into #1.