gcc-runtime, libgcc: Symlink c++ header and startup files in target_triplet for SDK use
We build SDKs such that gcc-cross-candian is built for only one
target *-*-linux and then use -muclibc or -mmusl to let it compile
code for other libc variants. This works fine when libc = glibc
however it does not work for c++ programs when libc != glibc since
there are c++ headers installed under ${includedir}/c++/${BINV}/${TARGET_SYS}
which is fine when gcc-runtime and gcc-cross-candian uses same --target options
gxx includedir searches in right triplet, but it fails with musl/uclibc
since gcc will look for glibc based triplet but gcc-runtime will install
them under musl/uclibc triplet.
This patch symlinks the musl/uclibc triplet to glibc triplet when libc != glibc
