rust: add some kludges to better support NetBSD HEAD-llvm

1) bootstrap rustc adds -lgcc_s when linking
-> Dropped with a BUILDLINK_TRANSFORM

2) bootstrap rustc has shared linkage to libgcc_s.so.1
-> Until upstream changes this to static linkage, we look for
  libgcc_s.so.1 in ${FILESDIR} where the user must place it manually.

3) newly built rustc adds -lstdc++ instead of -lc++ when linking llvm
-> fixed with patch-src_librustc__llvm_build.rs

4) newly built rustc adds -lgcc_s when linking
-> fixed with patch-src_libunwind_build.rs

(tnn)