[gdb/symtab] Make comp_unit_head.length private

Make comp_unit_head.length private, to enforce using accessor functions.

Replace accessor function get_length with get_length_with_initial and
get_length_without_initial, to make it explicit which variant we're using.

Tested on x86_64-linux.

PR symtab/29343
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29343