efi_loader: replace a u16_strdup with alloc + memcpy

Heinrich reports that on RISC-V unaligned access is emulated by OpenSBI
which is very slow.  Performance wise it's better if we skip the calls
to u16_strdup() -- which in turn calls u16_strsize() and just allocate/copy the
memory directly.  The access to dp.length may still be unaligned, but that's
way less than what u16_strsize() would do

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Use malloc() instead of calloc().
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>