Disable -Wc++-keyword because of false positives

[Leont]

This warning, a child category of -Wc++-compat, was causing a lot of false positives of the following variety because of a clang 21 issue, as reported in #24164:

In file included from op.c:163:
In file included from ./perl.h:6228:
./proto.h:6124:26: warning: identifier 'wchar_t' conflicts with a C++ keyword [-Wc++-keyword]
 6124 | Perl_mbtowc_(pTHX_ const wchar_t *pwc, const char *s, const Size_t len);
      |                          ^
1 warning generated.

• This set of changes does not require a perldelta entry.

[jkeenan]

This will, in fact, eliminate the -Wc++-keyword build-time warnings using clang21 reported in #24164.

$ report-build-warnings cf5ae39493.freebsd.threaded.clang21.maketp.output.txt.gz
File:  cf5ae39493.freebsd.threaded.clang21.maketp.output.txt.gz

  Wc++-keyword                              73
  Wimplicit-void-ptr-cast                    4

$ report-build-warnings b9b8d7aecf.freebsd.threaded.clang21.maketp.output.txt.gz
File:  b9b8d7aecf.freebsd.threaded.clang21.maketp.output.txt.gz

  Wimplicit-void-ptr-cast                    4