scons: Add a Transform() for when linking shared libraries.

Change-Id: I7ddba0cc7be559633328011c1c7e2282f509b78c
Reviewed-on: https://gem5-review.googlesource.com/2943
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>

SConstruct

@@ -648,6 +648,7 @@ else:
     main['SWIGCOMSTR']      = Transform("SWIG")
     main['ARCOMSTR']        = Transform("AR", 0)
     main['LINKCOMSTR']      = Transform("LINK", 0)
+    main['SHLINKCOMSTR']    = Transform("SHLINK", 0)
     main['RANLIBCOMSTR']    = Transform("RANLIB", 0)
     main['M4COMSTR']        = Transform("M4")
     main['SHCCCOMSTR']      = Transform("SHCC")