Provide default permissions if section has no permission flags

Provide default permissions if section has no permission flags

The permissions that a capability to an object should end up with is
based on the section it should point into.  With symbols that point into
SHN_ABS sections we have nothing to base the permissions on (since these
sections don't have associated permission flags).

For the moment we are making a default of choosing Read-Write
permissions and warning the user about it.  The permissions match what
Morello LLD currently does (from observation).
When Morello linkers use the symbol type to determine whether a
capability should have executable permissions or not, this should end up
being able to handle all uses (since STT_FUNC would get RX perms while
everything else gets RW perms).

In the only case we know of in the GNU team the symbol ends up with
zero-size anyway, so the choice of Read-Write doesn't seem too lax.
(Having zero-size is fine for the use-case we know of in glibc, since
that use case simply checks if the address of the symbol is non-zero.
Hence we have no need as yet to dereference the symbol).

The use case we know about are the `_nl_current_<LANG>_used` symbols
defined with `_NL_CURRENT_DEFINE` in the locale/lc-<lang>.c files in
statically linked glibc.  If any case that requires non-zero size or
different permissions becomes important then something more will be
required across the toolchain.