gallium/targets: use c++ compiler for linking

gallium/targets: use c++ compiler for linking

As pointed out by Michel Dänzer, gcc -lstdc++ doesn't work on all systems,
because it may require other libraries which are only pulled in implicitly
by g++. And libstdc++ is available only with GNU compiler.

Use c++ compiler for linking and remove redundant LDFLAGS += -lstdc++
all over the tree.