lang/gcc15-devel: fix build after base d08296c7ab0d

After base d08296c7ab0d7bb259bf7b8cdf9ffb819c1929ab ("libc: Implement
qualifier-preserving standard library functions"), lang/gcc15-devel
fails to build with an error like:

In file included from /wrkdirs/usr/ports/lang/gcc15-devel/work/.build/gcc/include-fixed/stdio.h:52,
                 from cp-demangle.c:109:
/wrkdirs/usr/ports/lang/gcc15-devel/work/gcc-15-20260724/libstdc++-v3/../include/libiberty.h:225:14: error: expected identifier or '(' before '_Generic'
  225 | extern void *memrchr(const void *, int, size_t);
      |              ^~~~~~~

This is because memrchr is now a macro, similar to the other functions
in <string.h>, and libiberty.h attempts to incorrectly redeclare it.

In turn, cp-demangle.c includes libiberty.h, but does not include the
config.h generated for libiberty.h, instead using the config.h generated
for libstdc++.

The problem is really with the way upstream creates its config.h
headers: they should either make libiberty.h automatically include its
own config.h file, or rename that one to libiberty-config.h, so it can
be included at the same time as the libstdc++ config.h header.

The easiest way to fix it is to add a '!defined(FreeBSD)'
conditional to the #if statement that attempts to declare the
incompatible memrchr.

PR: 297627
Approved by: salvadore (maintainer)
MFH: 2026Q3