[gdb/go] Fix literal truncation

Make sure we error out on overflow instead of truncating in all cases.

The current implementation of parse_number contains a comment about PR16377,
but that's related to C-like languages.  In absence of information of whether
the same fix is needed for go, take the conservative approach and keep
behaviour for decimals unchanged.

Tested on x86_64-linux, with a build with --enable-targets=all.