selftests: net/{lib,openvswitch}: extend CFLAGS to keep options from environment

selftests: net/{lib,openvswitch}: extend CFLAGS to keep options from environment

Package build environments like Fedora rpmbuild introduced hardening
options (e.g. -pie -Wl,-z,now) by passing a -spec option to CFLAGS
and LDFLAGS.

Some Makefiles currently override CFLAGS but not LDFLAGS, which leads
to a mismatch and build failure, for example:
  /usr/bin/ld: /tmp/ccd2apay.o: relocation R_X86_64_32 against
    `.rodata.str1.1' can not be used when making a PIE object; recompile with -fPIE
  /usr/bin/ld: failed to set dynamic section sizes: bad value
  collect2: error: ld returned 1 exit status
  make[1]: *** [../../lib.mk:222: tools/testing/selftests/net/lib/csum] Error 1

openvswitch/Makefile CFLAGS currently do not appear to be used, but
fix it anyway for the case when new tests are introduced in future.

Fixes: 1d0dc857b5d8 ("selftests: drv-net: add checksum tests")
Signed-off-by: Jan Stancek <jstancek@redhat.com>
Acked-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/3d173603ee258f419d0403363765c9f9494ff79a.1737635092.git.jstancek@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>