rust: fix computation of library directory

Using a rustup-based toolchain fails the "rust/2 sharedlib" test for me:

./prog: error while loading shared libraries: libstd-211931512faabf29.so: cannot open shared object file: No such file or directory

This happens because recent rustup places the standard library under
SYSROOT/lib/rustlib/TARGET/lib.  Retrieve the right directory using
"--print target-libdir".  This also provides a more accurate version
for rustc installed in /usr.

Before:
  $ echo $(/usr/bin/rustc --print sysroot)/lib
  /usr/lib

After:
  $ /usr/bin/rustc --print target-libdir
  /usr/lib/rustlib/x86_64-unknown-linux-gnu/lib

While at it, cache the value to avoid repeated process invocation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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