src/cmd/compile/internal/gc: remove now unnecessary restriction on float exponent parsing

https://go-review.googlesource.com/#/c/13778/ fixed this issue in math/big.
Remove restriction in compiler.

Fixes #11326.

Change-Id: I1429d0dd0d79431706c65616413373fff58f081e
Reviewed-on: https://go-review.googlesource.com/14830


Reviewed-by: Rob Pike <r@golang.org>