Fix type mismatch calculating new line program offset in debugedit.c.

Fix type mismatch calculating new line program offset in debugedit.c.

edit_dwarf2 calculates the (new) offset in the line program by
taking the difference between the old and new idx, which are of type
size_t (unsigned), plus the size_diff of the header given as ssize_t
(signed), and adding that to the current r_offset, which is an Elf64_Addr
(unsigned). On 64bit architectures, where the size of Elf64_Addr and
ssize_t are the same this isn't a problem. But on 32bit architectures,
where the size of ssize_t is smaller than Elf64_Addr the smaller signed
result gets promoted to an unsigned long first causing issues if the
size_diff was negative.

This would have been caught by gcc -Wsign-conversion

warning: conversion to ‘long unsigned int’ from ‘ssize_t’ {aka ‘long int’}
may change the sign of the result

But enabling this by default gives a lot of false positives.

Found and fixed by Richard Biener <rguenther@suse.de>.