fmt: support sharp flag for float and complex value printing

fmt: support sharp flag for float and complex value printing

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/37051
Reviewed-by: Rob Pike <r@golang.org>