sgi/install_prerequisites.sh: refactor install_libfdt

install_gcc_toolchain function is called only for x86_64 host machines.
Since install_libfdt function call is inside install_gcc_toolchain
function, it won't be called for aarch64 host machine. Hence move out
install_libfdt function call outside. Also, use the parameter passed
to the install_libfdt function only for x86_64 host as the aarch64 host
uses preinstalled gcc.

Signed-off-by: Vijayenthiran <vijayenthiran.subramaniam@arm.com>
Change-Id: Ib8b3d1ffb9d28874e4b59da0960c00a4e06f219d