x86inc: Add debug symbols indicating sizes of compiled functions

Some debuggers/profilers use this metadata to determine which function a
given instruction is in; without it they get can confused by local labels
(if you haven't stripped those). On the other hand, some tools are still
confused even with this metadata. e.g. this fixes `gdb`, but not `perf`.

Currently only implemented for ELF.

Signed-off-by: Anton Khirnov <anton@khirnov.net>