zos: build with -DZOSLIB_OVERRIDE_CLIB to override creat

The overridden creat() in zoslib, enabled by the subject macro, creates
a file as ISO8859-1 and tags it as text, so user no longer has to chtag
the generated build files manually, as was previously instructed in the
initial z/OS port (commit 45aa842fb4). Note: the macro also overrides
other C runtime library functions (e.g. open, fopen, pipe, etc.), but
these continue to function correctly.

The only file that will still be tagged as mixed binary and text (at
least in V8's gn-generated files) is toolchain.ninja (m ISO8859-1
instead of t ISO8859-1), because of the std::ios_base::binary in
NinjaToolchainWriter::RunAndWriteFile's std::ofstream open(). However,
zopen's vim, grep and ninja can still read it fine.

zoslib: https://github.com/ibmruntimes/zoslib
zopen: https://github.com/ZOSOpenTools/

Change-Id: Iaf3d8e7f622bdf7d9de5db4b92380ddf93ae9155
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16280
Reviewed-by: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Takuto Ikuta <tikuta@google.com>
diff --git a/build/gen.py b/build/gen.py
index 3364cb9..a463920 100755
--- a/build/gen.py
+++ b/build/gen.py
@@ -522,6 +522,7 @@
       cflags.append('-Wno-unused-function')
       cflags.append('-D_OPEN_SYS_FILE_EXT')
       cflags.append('-DPATH_MAX=1024')
+      cflags.append('-DZOSLIB_OVERRIDE_CLIB')
 
     if platform.is_posix() and not platform.is_haiku():
       ldflags.append('-pthread')