build: add option to enable LTO

build: add option to enable LTO

This patch adds an option to enable link time optimization.  In addition
to LTO option itself (-flto) fat-lto-objects are being used.  This is
because during the build pmdinfogen scans the generated ELF objects to
find this_pmd_name* symbol in symbol table.  Without fat-lto-objects gcc
produces ELF only with extra symbols for internal use during linking.

Signed-off-by: Andrzej Ostruszka <aostruszka@marvell.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>