sim: Increase jmpbuf size

Increase the size of the jmpbuf to accomodate other architectures.
Unfortunately, the size of this is not available in the libc crate.
Increase this so encompass any platforms we wish to support, including
aarch64 on both Linux and MacOS.

Increasing an array beyond 32 means there is no default offered, so
implement this manually.

Signed-off-by: David Brown <david.brown@linaro.org>