Fix rust-native build issue when debug is enabled

When DEBUG_BUILD is set for building rust-native, it generated
error as follows:-
=========================================================
Building : rustdoc, rustdoc-json-types
error[E0463]: can't find crate for `rustc_llvm`
which `rustc_driver` depends on
  --> src/librustdoc/lib.rs:37:1
   |
37 | extern crate rustc_driver;
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^ can't find crate
=========================================================
On analyzing the issue, it was found that rustc_llvm crate was present
at required path. However, it was very huge due to the presence of
debugging information. Hence, it was somehow not recognized as a valid
crate. The following patch removes the debug information from
rust-llvm-native which is built prior to rust-native but retains debug
information as required in rust-native binaries.

Signed-off-by: Pgowda <pgowda.cve@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>