llext: adjust memory permissions on MMU systems

By default, normal memory is set readable+writable and not executable.
Adjust those permissions according to each region:

	LLEXT_MEM_TEXT                  --> K_MEM_PERM_EXEC
	LLEXT_MEM_DATA, LLEXT_MEM_BSS   --> K_MEM_PERM_RW
	LLEXT_MEM_RODATA                --> K_MEM_PERM_RO aka 0

This must be done only at the end of an LLEXT load operation as memory
needs to remain RW for reloc processing, etc.

And while at it, flush/invalidate the cache accordingly.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>