GCN, libstdc++: '#define _GLIBCXX_USE_WEAK_REF 0' [PR119369]

GCN, libstdc++: '#define _GLIBCXX_USE_WEAK_REF 0' [PR119369]

This fixes a few hundreds of compilation/linking FAILs (similar to PR69506),
where the GCN/LLVM 'ld' reported:

    ld: error: relocation R_AMDGPU_REL32_LO cannot be used against symbol '_ZGTtnam'; recompile with -fPIC
    >>> defined in [...]/amdgcn-amdhsa/./libstdc++-v3/src/.libs/libstdc++.a(cow-stdexcept.o)
    >>> referenced by cow-stdexcept.cc:259 ([...]/libstdc++-v3/src/c++11/cow-stdexcept.cc:259)
    >>>               cow-stdexcept.o:(_txnal_cow_string_C1_for_exceptions(void*, char const*, void*)) in archive [...]/amdgcn-amdhsa/./libstdc++-v3/src/.libs/libstdc++.a

    ld: error: relocation R_AMDGPU_REL32_HI cannot be used against symbol '_ZGTtnam'; recompile with -fPIC
    >>> defined in [...]/amdgcn-amdhsa/./libstdc++-v3/src/.libs/libstdc++.a(cow-stdexcept.o)
    >>> referenced by cow-stdexcept.cc:259 ([...]/source-gcc/libstdc++-v3/src/c++11/cow-stdexcept.cc:259)
    >>>               cow-stdexcept.o:(_txnal_cow_string_C1_for_exceptions(void*, char const*, void*)) in archive [...]/amdgcn-amdhsa/./libstdc++-v3/src/.libs/libstdc++.a

    [...]

..., which is:

    $ c++filt _ZGTtnam
    transaction clone for operator new[](unsigned long)

..., and similarly for other libitm symbols.

However, the affected test cases, if applicable, then run into execution test
FAILs, due to PR119369
"GCN: weak undefined symbols -> execution test FAIL, 'HSA_STATUS_ERROR_VARIABLE_UNDEFINED'".

	PR target/119369
	libstdc++-v3/
	* config/cpu/gcn/cpu_defines.h: New.
	* configure.host [GCN] (cpu_defines_dir): Point to it.