bpf/docs: Update vmtest docs for static linking

Dynamic linking when compiling on the host can cause issues when the
libc version does not match the one in the VM image. Update the
docs to explain how to do this.

Before:
  ./vmtest.sh -- ./test_progs -t test_ima
  ./test_progs: /usr/lib/libc.so.6: version `GLIBC_2.33' not found (required by ./test_progs)

After:

  LDLIBS=-static ./vmtest.sh -- ./test_progs -t test_ima
  test_ima:OK
  Summary: 1/0 PASSED, 0 SKIPPED, 0 FAILED
Reported-by: "Geyslan G. Bem" <geyslan@gmail.com>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220307133048.1287644-1-kpsingh@kernel.org

tools/testing/selftests/bpf/README.rst

@@ -32,6 +32,14 @@ For more information on about using the script, run:
 
   $ tools/testing/selftests/bpf/vmtest.sh -h
 
+In case of linker errors when running selftests, try using static linking:
+
+.. code-block:: console
+
+  $ LDLIBS=-static vmtest.sh
+
+.. note:: Some distros may not support static linking.
+
 .. note:: The script uses pahole and clang based on host environment setting.
           If you want to change pahole and llvm, you can change `PATH` environment
           variable in the beginning of script.