cmake/compiler/gcc: use -flto only for gcc >= 4.5.0

Fixes commit 26c8776c ("buildsystem: Add an option to enable LTO")

Fixes the following error when compiling offsets.c

  xt-xcc ERROR parsing -fno-lto:  unknown flag

LTO options were introduced in gcc 4.5.0, compare:

https://gcc.gnu.org/onlinedocs/gcc-4.4.6/gcc/Option-Summary.html
https://gcc.gnu.org/onlinedocs/gcc-4.5.0/gcc/Option-Summary.html



Signed-off-by: Marc Herbert <marc.herbert@intel.com>