ANDROID: enable LLVM_IAS=1 for clang's integrated assembler for x86_64

Step 8 of:
https://android.googlesource.com/platform/prebuilts/clang/host/linux-x86/+/master/BINUTILS_KERNEL_DEPRECATION.md

Bug: 141693040
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I9d1621f6484c0402a7518ffb12a3f8f3815f43a9
diff --git a/build.config.x86_64 b/build.config.x86_64
index d5a358c..aa0c49e 100644
--- a/build.config.x86_64
+++ b/build.config.x86_64
@@ -1,5 +1,6 @@
 ARCH=x86_64
 
+LLVM_IAS=1
 CROSS_COMPILE=x86_64-linux-gnu-
 LINUX_GCC_CROSS_COMPILE_PREBUILTS_BIN=prebuilts/gas/linux-x86