librustc: add LLVM LDFLAGS to deps

This commit let librustc automatically pickup LDFLAGS dependencies
inherited from LLVM, which may otherwise result in undefined
references to external symbols under certain linking environment.

A symptom of this issue is eg. a failure when trying to link against
librustc (due to unresolved ffi_* symbols), while using a system-wide
LLVM.

Signed-off-by: Luca Bruno <lucab@debian.org>