Fix asm compiler flags change from cmake 0.1.44

cmake-rs@8141f0e changed the logic for handling asm compiler flags.
This change was pulled in with the cmake 0.1.42 -> 0.1.44 update.

This introduced two new flags to the LLVM build, breaking it:
"-DCMAKE_ASM_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64"
"-DCMAKE_ASM_COMPILER=/usr/bin/cc"

This patch should resolve the breakage by handling it in bootstrap.

src/bootstrap/native.rs

@@ -454,7 +454,8 @@ fn configure_cmake(
             }
         }
         cfg.define("CMAKE_C_COMPILER", sanitize_cc(cc))
-            .define("CMAKE_CXX_COMPILER", sanitize_cc(cxx));
+            .define("CMAKE_CXX_COMPILER", sanitize_cc(cxx))
+            .define("CMAKE_ASM_COMPILER", sanitize_cc(cc));
     }
 
     cfg.build_arg("-j").build_arg(builder.jobs().to_string());