A 16bit binary exponent permits a constant range covering roughly the range
from 7e-9865 to 7e9863 which is more than enough for any practical and
hypothetical constant arithmetic.

Furthermore, until recently cmd/compile could not handle very large exponents
correctly anyway; i.e., the chance that any real programs (but for tests that
explore corner cases) are affected are close to zero.

Finally, restricting the minimum supported range significantly reduces the
implementation complexity in an area that hardly matters in reality for new
or alternative spec-compliant implementations that don't or cannot rely on
pre-existing arbitratry precision arithmetic packages that support a 32bit
exponent range.

This is technically a language change but for the reasons mentioned above
this is unlikely to affect any real programs, and certainly not programs
compiled with the gc or gccgo compilers as they currently support up to
32bit exponents.

Fixes #13572.

Change-Id: I970f919c57fc82c0175844364cf48ea335f17d39
Reviewed-on: https://go-review.googlesource.com/17711Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>