Makefile.uk: Use -ffreestanding as internal build option

Makefile.uk: Use -ffreestanding as internal build option

As a standard C library (libc), newlib is to be built in a freestanding
environment (i.e. not relying on a libc). For that, build internal
newlib source code files with the `-ffreestanding` option. Otherwise, a
hosted environment will be used; in a hosted environment, certain
optimizations may be enabled by the compiler, and they may cause issues
(such as an infinite set of calls).

Signed-off-by: Razvan Deaconescu <razvand@unikraft.io>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Approved-by: Marc Rittinghaus <marc.rittinghaus@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #33