Reduce number of unnecessary sections in our executable

This patch resolves issue #26. As you can see with "objdump -h
build/release/loader.elf", our executable had over a thousand (!)
separate sections, most of them should really be merged.
We already started doing this in arch/x64/loader.ld, but didn't
complete the work.

This patch merges all the ".gcc_except_table.*" sections into one,
and all the ".data.rel.ro.*" sections into one. After this merge,
we are left with just 52 sections, instead of more than 1000.

The default linker script (run "ld --verbose" to see it) also does
similar merges, so there's no reason why we shouldn't.

By reducing the number of ELF sections (each comes with a name, headers,
etc.), this patch also reduces the size of our loader-stripped.elf
by about 140K.

Fixes #26.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>