BaseTools/header.makefile: revert gcc-8 "-Wno-xxx" options on OSX

BaseTools/header.makefile: revert gcc-8 "-Wno-xxx" options on OSX

I recently added the gcc-8 specific "-Wno-stringop-truncation" and
"-Wno-restrict" options to BUILD_CFLAGS, both for "Darwin" (XCODE5 /
clang, OSX) and otherwise (gcc, Linux / Cygwin).

I also regression-tested the change with gcc-4.8 on Linux -- gcc-4.8 does
not know either of the (gcc-8 specific) "-Wno-stringop-truncation" and
"-Wno-restrict" options, yet the build completed fine (by GCC design).

Regarding OSX, my expectation was that

- XCODE5 / clang would either recognize these warnings options (because
  clang does recognize most -W options of gcc),

- or, similarly to gcc, clang would simply ignore the "-Wno-xxx" flags
  that it didn't recognize.

Neither is the case; the new flags have broken the BaseTools build on OSX.
Revert them (for OSX only).

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Reported-by: Liming Gao <liming.gao@intel.com>
Fixes: 1d212a83df0eaf32a6f5d4159beb2d77832e0231
Fixes: 9222154ae7b3eef75ae88cdb56158256227cb929
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>