[gn build] Fix up win/x86 flags and add stage2_unix_x86

[aeubanks]

No description provided.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder openmp-offload-libc-amdgpu-runtime running on omp-vega20-1 while building llvm at step 10 "Add check check-offload".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/73/builds/6821

Here is the relevant piece of the build log for the reference

Step 10 (Add check check-offload) failure: 1200 seconds without output running [b'ninja', b'-j 32', b'check-offload'], attempting to kill
...
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug47654.cpp (866 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug49779.cpp (867 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/test_libc.cpp (868 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug50022.cpp (869 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/wtime.c (870 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu :: offloading/bug49021.cpp (871 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu :: offloading/std_complex_arithmetic.cpp (872 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/complex_reduction.cpp (873 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/bug49021.cpp (874 of 879)
PASS: libomptarget :: x86_64-unknown-linux-gnu-LTO :: offloading/std_complex_arithmetic.cpp (875 of 879)
command timed out: 1200 seconds without output running [b'ninja', b'-j 32', b'check-offload'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1237.226966