The current implementation accepted binary exponents but restricted
them to 'p'. This change permits both 'p' and 'P'.

R=Go1.13

Updates #29008.

Change-Id: I7a89ccb86af4438f17b0422be7cb630ffcf43272
Reviewed-on: https://go-review.googlesource.com/c/159297Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>