5l: document -F, force it on old ARMs

Fixes #1341.

R=ken2, r
CC=golang-dev
https://golang.org/cl/4006045

src/cmd/5l/doc.go

@@ -20,6 +20,9 @@ Original options are listed in the link above.
 
 Options new in this version:
 
+-F
+	Force use of software floating point.
+	Also implied by setting GOARM=5 in the environment.
 -L dir1 -L dir2
 	Search for libraries (package files) in dir1, dir2, etc.
 	The default is the single location $GOROOT/pkg/$GOOS_arm.

src/cmd/5l/obj.c

@@ -69,6 +69,7 @@ void
 main(int argc, char *argv[])
 {
 	int c, i;
+	char *p;
 
 	Binit(&bso, 1, OWRITE);
 	cout = -1;
@@ -80,6 +81,10 @@ main(int argc, char *argv[])
 	INITDAT = -1;
 	INITRND = -1;
 	INITENTRY = 0;
+	
+	p = getenv("GOARM");
+	if(p != nil && strcmp(p, "5") == 0)
+		debug['F'] = 1;
 
 	ARGBEGIN {
 	default: