[Perl/perl5] f9760c: win32/GNUmakefile: support CFG=DebugSymbols for gcc

[Tony Cook via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: f9760c232c84c88fd9393f10cf89f9f71818e952
      
https://github.com/Perl/perl5/commit/f9760c232c84c88fd9393f10cf89f9f71818e952
  Author: Tony Cook <t...@develop-help.com>
  Date:   2023-08-07 (Mon, 07 Aug 2023)

  Changed paths:
    M win32/GNUmakefile

  Log Message:
  -----------
  win32/GNUmakefile: support CFG=DebugSymbols for gcc

This is already supported for MSVC