llext: add storage-specific buffer loaders

This patch adds three new macros to initialize an llext_buf_loader
structure with a specific storage type. The existing LLEXT_BUF_LOADER
macro is discouraged, since its storage type is not explicitly defined
in the code but relies on the CONFIG_LLEXT_STORAGE_WRITABLE option.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>