valgrind: Don't disable optimizations for valgrind builds

When we plan to use valgrind, we need to build passt a bit differently:
  * We need debug symbols so that valgrind can match problems it finds to
    meaningful locations
  * We need to allow additional syscalls in the seccomp filter, because
    valgrind's wrappers need them

Currently we also disable optimization (-O0).  This is unfortunate, because
it will make valgrind tests even slower than they'd otherwise be.  Worse,
it's possible that the asm behaviour without optimization might be
different enough that valgrind could miss a use of uninitialized variable
or other fault it would detect.

I suspect this was originally done because without it inlining could mean
that suppressions we use don't reliably match the places we want them to.
Alas, it turns out this is true even with -O0.  We've now implemented a
more robust workaround for this (explicit ((noinline)) attributes when
compiled with -DVALGRIND).  So, we can re-enable optimization for valgrind
builds, making them closer to regular builds.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Makefile

@@ -128,7 +128,7 @@ qrap: $(QRAP_SRCS) passt.h
 valgrind: EXTRA_SYSCALLS += rt_sigprocmask rt_sigtimedwait rt_sigaction	\
 			    getpid gettid kill clock_gettime mmap	\
 			    munmap open unlink gettimeofday futex
-valgrind: FLAGS:=-g -O0 $(filter-out -O%,$(FLAGS)) -DVALGRIND
+valgrind: FLAGS += -g -DVALGRIND
 valgrind: all
 
 .PHONY: clean