build: Use -ffreestanding

this is a stand-alone/baremetal application, therefore demanding
-ffreestanding would help it compile with hosted toolchains e.g. ( linux
toolchains ), it also ensures that it won't be using platform
optimizations like inlining mem* str* functions which gcc might decide
especially with wrapper string functions in opensbi code

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>