mips:  Adjust main & irq stack sizes to fit in IRAM on all targets

New amounts:

7.50K main stack
0.75K irq stack

Prior values of 8K+1K overflowed IRAM by ~660 bytes on Onda 7x7 targets,
but worked on the xDuoo X3.

(The discrepancy is due to the Onda targets having more LCD code shoved
 into IRAM.)

Change-Id: I16fcfae3c5f3e36db688dfa9167b620584e79df8
2 files changed