efi_loader: make .data section of *_efi.so files RW

When building with binutils 2.39 warnings

    *_efi.so has a LOAD segment with RWX permissions

occur.

Use SHF_WRITE | SHF_ALLOC as section flags for the .data section.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>