elf: Calculate symbol size if needed

elf: Calculate symbol size if needed

Symbols with a size of 0 are unusable for the disassembler.

Example:

While running an arm linux kernel, no symbolic names are
used in qemu.log when the cpu is executing an assembler function.

Assume that the size of such symbols is the difference to the
next symbol value.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>