cheri: elf: Use RW permissions for l_ld when needed

cheri: elf: Use RW permissions for l_ld when needed

The dynamic section of an executable needs to be written to set the
DT_DEBUG entry for debuggers (unless the target has some other place
to store r_debug). For this reason we make l_ld writable whenever
the dynamic section is writable.

The ld.so l_ld is kept RX, since it does not have DT_DEBUG.
(Note: relocating the dynamic section is not allowed on cheri and
that's the only other reason glibc would write to it.)