execline: wrap unconditionally; strip

I don't think there's any situation in which an unwrapped execlineb is
useful -- if you want to use different versions of the execlineb tool
it'll still prefer ones in PATH.  At the same time, implementing the
wrapper in this way, as a series of two derivations, meant that we
didn't get stdenv goodness for the wrapper.  This meant that, for
example, the wrapper was not stripped, and so execline ended up with
runtime dependencies on gcc and the Linux headers.  I don't want to
have to reimplement this sort of stuff when it's already in stdenv,
and so it makes much more sense to create the wrapper in the
mkDerivation call, where all of stdenv's normal magic will find it.

pkgs/tools/misc/execline/execlineb-wrapper.c

@@ -1,3 +1,10 @@
+/*
+ * A wrapper around execlineb, which provides all execline
+ * tools on execlineb’s PATH.
+ * It is implemented as a C program, because on non-Linux,
+ * nested shebang lines are not supported.
+ */
+
 #include <stdlib.h>
 #include <string.h>