Attempt to fix the Sparc version of StgRun.  Since
RESERVED_STACK_SPACE increased beyond the size of a 13-bit immediate,
the Sparc code has been broken.

We try to fix this with some imaginative gcc inline assembly
constraints.  If this breaks again, I'll try recoding it directly in
assembler.

While I'm here, pull in some of the comments from the old pre-4.00 RTS
and add some new comments.