Added an alternate form of printing floats and complex values
by specifying the sharp flag.

Output formatted using the the verbs v, e, E, f, F, g and G in
combination with the sharp flag will always include a decimal point.

The alternate form specified by the sharp flag for %g and %G verbs
will not truncate trailing zeros and assume a default precision of 6.

Fixes #18857.

Change-Id: I4d776239e06d7a6a90f2d8556240a359888cb7c3
Reviewed-on: https://go-review.googlesource.com/37051Reviewed-by: Rob Pike <r@golang.org>