Change STACK_OVERFLOW_CHECK to 1 by default, and not 2.

[kripken]

Change the default value of STACK_OVERFLOW_CHECK in builds with ASSERTIONS
from 2 to 1. This means that plain debug builds (-O0, which enables
ASSERTIONS) do not have the most expensive stack checks on by default. You
can still add them with -s STACK_OVERFLOW_CHECK=2.

The most expensive checks seem excessive to run by default. I don't think any
platform does them. However, there is slightly more risk in wasm since we don't
have memory protection to give a guarantee of a trap. But the default stack
protections can be enough to detect that in most cases.

(The default protection, 1, writes a cookie and asserts the cookie is not overwritten
at various times. The expensive checks modify the wasm to make every stack
allocation check the limit.)

See #3043, this removes some work we were doing by default after link, so it
makes the link stage simpler.

[sbc100]

Nice!