[LibOS] Make sure LibOS doesn't have the executable stack flag

This ensures that the LibOS binary (`libsysdb.so`) is not incorrectly
marked as requiring executable stack.

Note that this doesn't have any visible effect right now: our ELF loader
does not respect the relevant program header (`GNU_STACK`), and under
SGX 1, we need to map all memory as RWX anyway.

Technical details: The linker clears the executable stack flag for a
binary only if none of the `.o` files require it. This is determined
automatically for C sources, but when compiling assembly, we need to
pass an extra `-Wa,--noexecstack` flag. This flag is already set
globally, but `vdso-data.o` is a Meson custom target, and doesn't use
global flags.

Signed-off-by: Paweł Marczewski <pawel@invisiblethingslab.com>

LibOS/shim/src/meson.build

@@ -121,6 +121,7 @@ vdso_data_o = custom_target('vdso-data.o',
     command: [
         cc.cmd_array(),
         cflags_libos,
+        '-Wa,--noexecstack',
         '-DVDSO_SO_FULL_PATH="@0@"'.format(vdso_so.full_path()),
         '-c',
         '@INPUT@',