sandbox: fix compiling of cpu/os.c

U-boot itself generally builds with -nostdinc.  This is because the
bootloader needs to be completely standalone.  In the sandbox arch
though, we need a little bit of code to glue the u-boot world to the
host operating system, and we need to be able to access the host
libc's headers in order to do so.

Currently, we're using -I/usr/include to workaround the global
-nostdinc, but that doesn't work for everyone and for all headers.
Instead, let's filter out -nostdinc when building the os.c code.

Without this patch, some distros hit errors such as:
---8<---
In file included from /usr/include/fcntl.h:27:0,
                 from os.c:22:
/usr/include/features.h:323:26: fatal error:
     bits/predefs.h: No such file or directory
--->8---

Signed-off-by: Andreas Bießmann <biessmann@corscience.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>