Building test_vdso_chacha currently leads to following issue:

  In file included from /home/chleroy/linux-powerpc/include/linux/limits.h:7,
                   from /opt/powerpc64-e5500--glibc--stable-2024.02-1/powerpc64-buildroot-linux-gnu/sysroot/usr/include/bits/local_lim.h:38,
                   from /opt/powerpc64-e5500--glibc--stable-2024.02-1/powerpc64-buildroot-linux-gnu/sysroot/usr/include/bits/posix1_lim.h:161,
                   from /opt/powerpc64-e5500--glibc--stable-2024.02-1/powerpc64-buildroot-linux-gnu/sysroot/usr/include/limits.h:195,
                   from /opt/powerpc64-e5500--glibc--stable-2024.02-1/lib/gcc/powerpc64-buildroot-linux-gnu/12.3.0/include-fixed/limits.h:203,
                   from /opt/powerpc64-e5500--glibc--stable-2024.02-1/lib/gcc/powerpc64-buildroot-linux-gnu/12.3.0/include-fixed/syslimits.h:7,
                   from /opt/powerpc64-e5500--glibc--stable-2024.02-1/lib/gcc/powerpc64-buildroot-linux-gnu/12.3.0/include-fixed/limits.h:34,
                   from /tmp/sodium/usr/local/include/sodium/export.h:7,
                   from /tmp/sodium/usr/local/include/sodium/crypto_stream_chacha20.h:14,
                   from vdso_test_chacha.c:6:
  /opt/powerpc64-e5500--glibc--stable-2024.02-1/powerpc64-buildroot-linux-gnu/sysroot/usr/include/bits/xopen_lim.h:99:6: error: missing binary operator before token "("
     99 | # if INT_MAX == 32767
        |      ^~~~~~~
  /opt/powerpc64-e5500--glibc--stable-2024.02-1/powerpc64-buildroot-linux-gnu/sysroot/usr/include/bits/xopen_lim.h:102:7: error: missing binary operator before token "("
    102 | #  if INT_MAX == 2147483647
        |       ^~~~~~~
  /opt/powerpc64-e5500--glibc--stable-2024.02-1/powerpc64-buildroot-linux-gnu/sysroot/usr/include/bits/xopen_lim.h:126:6: error: missing binary operator before token "("
    126 | # if LONG_MAX == 2147483647
        |      ^~~~~~~~

This is due to kernel include/linux/limits.h being included instead of
libc's limits.h.

This is because directory include/ is added through option -isystem so
it goes prior to glibc's include directory.

Replace -isystem by -idirafter.

But this implies that now tools/include/linux/linkage.h is included
instead of include/linux/linkage.h, so define a stub for
SYM_FUNC_START() and SYM_FUNC_END().
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>