llext: fix incorrect elf64_sym structure

The elf64_sym structure was defined as having the members order
as the elf32_sym structure, which is incorrect.

Use the correct layout as defined by the ELF specification
(see https://refspecs.linuxfoundation.org/elf/gabi4+/ch4.symtab.html

)

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>