Revert of [NaCl SDK] Add initial support for building the SDK with gn (patchset #6 id:100001 of https://codereview.chromium.org/1693053002/ )

Reason for revert:
Doesn't build with clang/win and looks nontrivial to fix:

https://build.chromium.org/p/chromium.fyi/builders/CrWinClang64%28dbg%29/builds/3655/steps/compile/logs/stdio

FAILED: ninja -t msvc -e environment.x64 -- E:\b\build\goma/gomacc.exe ../../third_party/llvm-build/Release+Asserts/bin/clang-cl.exe /nologo /showIncludes /FC @obj/native_client/src/trusted/validator/driver/ncval_new/ncval.obj.rsp /c ../../native_client/src/trusted/validator/driver/ncval.cc /Foobj/native_client/src/trusted/validator/driver/ncval_new/ncval.obj /Fdobj/native_client/src/trusted/validator/driver/ncval_new_cc.pdb
../../native_client/src/trusted/validator/driver/ncval.cc(359,36) :  error: ISO C++11 does not allow conversion from string literal to 'char *' [-Werror,-Wwritable-strings]
  while ((opt = getopt(argc, argv, "vd")) != -1) {
                                   ^

(and i'm guessing several more)

Original issue's description:
> [NaCl SDK] Add initial support for building the SDK with gn
>
> build_sdk.py still uses gyp.  I have a followup CL to
> convert that to gn.
>
> BUG=585999

TBR=binji@chromium.org,dpranke@chromium.org,bradnelson@chromium.org,sbc@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=585999

Review URL: https://codereview.chromium.org/1693213002

Cr-Commit-Position: refs/heads/master@{#375349}
4 files changed