Copy the correct libs when using local-rust-root

This fixes a segfault when configuring rust to use local-rust-root. The
libraries were renamed in the 0.6-0.7 transition, and the script was not copying
them all. I also removed the line referencing libcore (now libstd).
auto merge of #7586 : g5pw/rust/master, r=cmr

This updates the script in src/etc/local_stage0.sh to copy the correct libraries
to stage0.
Fix a warning when generating tests
auto merge of #7591 : alexcrichton/rust/fix-warnings, r=huonw

Closes #7589
vec: make vec_reserve_shared_actual private
add a temporary vector_exchange_malloc lang item
remove headers from exchange allocations