Only check for valid Morello bounds on non-exec syms

Only check for valid Morello bounds on non-exec syms

Capabilities pointing to symbols in SEC_CODE sections are given the
bounds of the entire PCC.  We ensure that the PCC bounds are padded and
aligned as needed in the linker.

Capabilities pointing to other symbols (e.g. in data sections) are given
the bounds of the symbol that they point to.  It is the responsibility
of the assembly generator (i.e. usually the compiler) to ensure these
bounds are correctly aligned and padded as necessary.

We emit a warning for imprecise bounds in the second case, until this
patch that warning also looked at the first case.  This was a mistake
and is rectified in this commit.