Make gcc move switch tables into .rodata instead of .text on i386/amd64

Make gcc move switch tables into .rodata instead of .text on i386/amd64

For C++, gcc has to make use of comdat sections instead
of .gnu.linkonce sections for this because
switch tables and functions would now end up
in different .gnu.linkonce sections. This can cause ld
to sometimes incorrectly discard the switch tables, which causes
linker errors. With comdat sections, making the switch table
and function sections belong together is more reliable.

ok deraadt@