GCN, nvptx: Define '_Unwind_RaiseException', '_Unwind_Resume_or_Rethrow'

GCN, nvptx: Define '_Unwind_RaiseException', '_Unwind_Resume_or_Rethrow'

This resolves GCN:

    ld: error: undefined symbol: _Unwind_RaiseException
    >>> referenced by eh_throw.cc:93 ([...]/source-gcc/libstdc++-v3/libsupc++/eh_throw.cc:93)
    >>>               eh_throw.o:(__cxa_throw) in archive /srv/data/tschwinge/amd-instinct2/gcc/build/submit-light-target_gcn/build-gcc/amdgcn-amdhsa/gfx908/libstdc++-v3/src/.libs/libstdc++.a
    [...]
    collect2: error: ld returned 1 exit status

..., and/or:

    ld: error: undefined symbol: _Unwind_Resume_or_Rethrow
    >>> referenced by eh_throw.cc:129 ([...]/source-gcc/libstdc++-v3/libsupc++/eh_throw.cc:129)
    >>>               eh_throw.o:(__cxa_rethrow) in archive /srv/data/tschwinge/amd-instinct2/gcc/build/submit-light-target_gcn/build-gcc/amdgcn-amdhsa/gfx908/libstdc++-v3/src/.libs/libstdc++.a
    [...]
    collect2: error: ld returned 1 exit status

..., and nvptx:

    unresolved symbol _Unwind_RaiseException
    collect2: error: ld returned 1 exit status

..., or:

    unresolved symbol _Unwind_Resume_or_Rethrow
    collect2: error: ld returned 1 exit status

For both GCN, nvptx, this each progresses ~25 'check-gcc-c++',
and ~10 'check-target-libstdc++-v3' test cases:

    [-FAIL:-]{+PASS:+} [...] (test for excess errors)

..., with (if applicable, for most of them):

    [-UNRESOLVED:-]{+PASS:+} [...] [-compilation failed to produce executable-]{+execution test+}

..., or some 'FAIL: [...] execution test' where these test cases now FAIL when
attempting to use these interfaces, or, if applicable, FAIL due to run-time
'GCC/nvptx: sorry, unimplemented: dynamic stack allocation not supported'.

	libgcc/
	* config/gcn/unwind-gcn.c (_Unwind_RaiseException)
	(_Unwind_Resume_or_Rethrow): New.
	* config/nvptx/unwind-nvptx.c (_Unwind_RaiseException)
	(_Unwind_Resume_or_Rethrow): Likewise.

(cherry picked from commit 54ab0f6785c006ddff056dab9c0240cddae82020)