valgrind: do not use host mpicc

Valgrind may use 'mpicc' (from openmi project) to compile libmpiwrap-*.so.
Without any option, it will detect and use 'mpicc' from host and generate
libmpiwrap-*.so for host:

  $ file target/usr/lib/valgrind/libmpiwrap-arm-linux.so
  target/usr/lib/valgrind/libmpiwrap-arm-linux.so: ELF 64-bit LSB  shared object,
  x86-64, version 1 (SYSV), dynamically linked, not stripped

We simply disable openmi support for valgrind.

Signed-off-by: Jérôme Pouiller <jezz@sysmic.org>
[Thomas: refactor with other unconditional CONF_OPTS.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>