libstdc++-v3: Update VTV vars for libtool link commands [PR99172]

libstdc++-v3: Update VTV vars for libtool link commands [PR99172]

This fixes PR 99172

Currently when GCC is configured with --enable-vtable-verify, the
libstdc++-v3 Makefiles add "-fvtable-verify=std
-Wl,-u_vtable_map_vars_start,-u_vtable_map_vars_end" to libtool link
commands. The "-fvtable-verify=std" piece causes alternate versions of
libtool (such as slibtool) to fail, unable to find "-lvtv" (GNU
libtool just removes that piece).

This patch updates the libstdc++-v3 Makefiles to not pass
"-fvtable-verify=std" to the libtool link commands.