fmt: make %F a synonym for %f
Rationale:
        It already is for scanning.
        It is accepted for complexes already, but doesn't work.
        It's analogous to %G and %E.
        C accepts it too, and we try to be roughly compatible.
Fixes #7518.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/77580044
diff --git a/src/pkg/fmt/format.go b/src/pkg/fmt/format.go
index 3835aa9..b0f4ad4 100644
--- a/src/pkg/fmt/format.go
+++ b/src/pkg/fmt/format.go
@@ -447,7 +447,7 @@
 			f.fmt_e32(r)
 		case 'E':
 			f.fmt_E32(r)
-		case 'f':
+		case 'f', 'F':
 			f.fmt_f32(r)
 		case 'g':
 			f.fmt_g32(r)
@@ -477,7 +477,7 @@
 			f.fmt_e64(r)
 		case 'E':
 			f.fmt_E64(r)
-		case 'f':
+		case 'f', 'F':
 			f.fmt_f64(r)
 		case 'g':
 			f.fmt_g64(r)