test: adjust issue11371 to fit in required precision

The language spec only requires that floating point values be
represented with 256 bits, which is about 1e75. The issue11371 test
was assuming that the compiler could represent 1e100. Adjusting the
test so that it only assumes 256 bits of precision still keeps the
test valid, and permits it to pass when using the gofrontend.

Change-Id: I9d1006e9adc9438277f4b8002488c912e5d61cc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/278116


Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>