[gcn] Don't default to building target-libstdc++-v3 [PR92713]

[gcn] Don't default to building target-libstdc++-v3 [PR92713]

... which hasn't been ported/fails to build when using newlib (with GCC commit
b73f69020f08208d2d969fcf8879bd294a6e3596 sources, and newlib commit
6d79e0a58866548f435527798fbd4a6849d05bc7, tag: newlib-3.3.0 sources):

    In file included from [...]/build-gcc-offload-amdgcn-amdhsa/amdgcn-amdhsa/libstdc++-v3/include/csetjmp:42,
                     from [...]/source-gcc/libstdc++-v3/include/precompiled/stdc++.h:42:
    [...]/source-gcc/newlib/libc/include/setjmp.h:15:6: error: variable or field 'longjmp' declared void
       15 | void longjmp (jmp_buf __jmpb, int __retval)
          |      ^~~~~~~
    [...]
    Makefile:1824: recipe for target 'amdgcn-amdhsa/bits/stdc++.h.gch/O2ggnu++0x.gch' failed
    make[3]: *** [amdgcn-amdhsa/bits/stdc++.h.gch/O2ggnu++0x.gch] Error 1

	PR target/92713
	* configure.ac ["${ENABLE_LIBSTDCXX}" = "default" && amdgcn*-*-*]
	(noconfigdirs): Add 'target-libstdc++-v3'.
	* configure: Regenerate.