* cannon: Fix GC emulation of Go programs

Improves Linux/MIPS32 emulation for Go programs that utilize the garbage
collector and goroutine scheduling.

This adds support for the following syscalls:

- getpid - used by the go scheduler
- clock_gettime - used by the go scheduler and for GC assists and to properly emulate
  time related operations such as `time.Sleep`.

Note on GC assists:

The Go GC relies on `clock_gettime` for GC "assists", whereby a mutator can perform a little bit
of GC without waiting for the scheduler to do so.
A monotonic clock (runtime.nanotime) is used to compute the current goroutine's compute budget.
By modeling a MIPS32 CPU that runs at some clock speed (ex: 10 MHz), we can provide a consistent
emulation of monotonic time needed by the Go runtime.
All other clock_gettime flags are handled as unimplemented syscalls.

* fix unsupported syscalls test

* fix some review comments

* address review comments

* update snapshots

* fuzz invalid memory proof

* reduce test runtime

* tweak realtime emulation

* reduce test runtime

* set a high timeout for heavy fuzz tests

* simplify fuzz

* fix heavy tests