Improvements to the statistical profiler.
* Add support for x86-64.
* Time spent in foreign functions was previously attributed to
"elsewhere". Now reported individually for all of them, and
"foreign function __open", "foreign function sin", etc.
* Walk the call stack to a depth of 8 instead of relying only
on the PC and RA from os-context-t (currently only on x86/x86-64).
Thus instead of just knowing that a lot of time is being spent
on (for example) bignum operations, we might find out where
the bignum operations are being initiated.
* Add a column for accrued time to the flat report.
* Add kludgy workaround for invalid frame-pointers in the
os-context-t structure causing segfaults.