llext: add support for SLID-based linking

This commit introduces support for an alternate linking method in the
LLEXT subsystem, called "SLID" (short for Symbol Link Identifier),
enabled by the CONFIG_LLEXT_EXPORT_BUILTINS_BY_SLID Kconfig option.

SLID-based linking uses a unique identifier (integer) to identify
exported symbols, instead of using the symbol name as done currently.
This approach provides several benefits:
 * linking is faster because the comparison operation to determine
   whether we found the correct symbol in the export table is now an
   integer compare, instead of a string compare
 * binary size is reduced as symbol names can be dropped from the binary
 * confidentiality is improved as a side-effect, as symbol names are no
   longer present in the binary

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