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

This reverts commit e9a55d97.

Re-enables the use of LLVM_IAS=1. This was disabled in aosp/1559459 due
to a single time-based test failure. This test failure is not
reproducible locally. The failure has never been observed on newer
kernel versions either. We should consider disabling this test if we
observe further flakes.

Test: 1. apply this patch
2. build kernel image
  $ BUILD_CONFIG=common/build.config.gki.x86_64 build/build.sh && \
    BUILD_CONFIG=common-modules/virtual-device/build.config.cuttlefish.x86_64 \
    build/build.sh
3. launch cuttlefish with custom kernel image
  $ launch_cvd -kernel_path=/path/to/out/android-4.19-stable/dist/bzImage \
    -initramfs_path=/path/to/out/android-4.19-stable/dist/initramfs.img &
4. run test
  $ atest vts_ltp_test_\
    x86:syscalls.nanosleep01_32bit#syscalls.nanosleep01_32bit -- --abi x86
Test was repeated 10 times locally, 0 failures observed.
Bug: 141693040
Bug: 178427746
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Change-Id: I6e93fafe0477df9c55cc6a0fa145660975927294