inteltool: Fixed building of position independent executables

inteltool: Fixed building of position independent executables

When building a position independent executable (PIE) EBX is used
internally by the compiler to generate position independent address
references so it cannot be used in the clobber list. Use the already
existing code for the Darwin plattform for that case, too -- it'll
preserve the EBX value.

Change-Id: Ief6d4872b8cd990856a0e8227a88bb228782aced
Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Reviewed-on: http://review.coreboot.org/209
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>