doc: Mention the target_samples and ld variables for fate configs

Signed-off-by: Martin Storsjö <martin@martin.st>

doc/fate.texi

@@ -128,10 +128,12 @@ cpu=
 cross_prefix=
 as=
 cc=
+ld=
 target_os=
 sysroot=
 target_exec=
 target_path=
+target_samples=
 extra_cflags=
 extra_ldflags=
 extra_libs=