MIPS: Don't use $t8 for calling entrypoints

When jumping to entrypoints, $t9 register must be used. Value of $gp
is calculated based on value from $t9 and it should contain the
address of the first instruction of the entrypoint.

This CL enables MIPS32 and MIPS64 to boot.

Change-Id: Idf0f7f479cceafabb88b1b087da190dbda86e1c3
2 files changed