[gdb/symtab] Fix overly large gdb-index file check for 32-bit

[gdb/symtab] Fix overly large gdb-index file check for 32-bit

Add a unit test which checks that write_gdb_index_1 will throw
an error when the size of the file would exceed the maximum value
capable of being represented by 'offset_type'.

The unit test fails on 32-bit systems due to wrapping overflow.  Fix this by
changing the type of total_len in write_gdbindex_1 from size_t to uint64_t.

Tested on x86_64-linux.

Co-Authored-By: Kevin Buettner <kevinb@redhat.com>
Approved-by: Kevin Buettner <kevinb@redhat.com>