-Wuninitialized on clang

[Jack Howarth]

The gdb-7.9 build on clang reports variable 'uoffset' is uninitialized
when used in the -Wuninitialized warnings...

./compile/compile-loc2c.c:729:6: warning: variable 'uoffset' is
uninitialized when used here [-Wuninitialized]
            uoffset += dwarf2_per_cu_text_offset (per_cu);
            ^~~~~~~
./compile/compile-loc2c.c:667:23: note: initialize the variable
'uoffset' to silence this warning
      uint64_t uoffset, reg;
                      ^
                       = 0