tox: Explicitly use -fno-omit-frame-pointer for ASAN/TSAN builds

Some compiler versions and distributions default to that out of the box,
while some others default to -fomit-frame-pointer. Since for sanitizers
it is important to have the frame-pointers to be able to retrieve
program tracebacks even without DWARF and faster(*), let's explicitly make
sure that what we build with comes with frame-pointers enabled.

(*) see e.g. https://github.com/google/sanitizers/wiki/AddressSanitizer#using-addresssanitizer
    for details.

tox.ini

@@ -42,16 +42,16 @@ basepython =
 setenv =
 # distutils take CFLAGS for both C and C++.
 # distutils use  CFLAGS also at link stage -> we don't need to set LDFLAGS separately.
-    tsan: CFLAGS=-g -fsanitize=thread
-    asan: CFLAGS=-g -fsanitize=address
+    tsan: CFLAGS=-g -fsanitize=thread   -fno-omit-frame-pointer
+    asan: CFLAGS=-g -fsanitize=address  -fno-omit-frame-pointer
 # XXX however distutils' try_link, which is used by numpy.distutils use only CC
 # as linker without CFLAGS and _without_ LDFLAGS, which fails if *.o were
 # compiled with -fsanitize=X and linked without that option. Work it around
 # with also adjusting CC.
 # XXX better arrange to pass CFLAGS to pygolang only, e.g. by adding --race or
 # --sanitize=thread to `setup.py build_ext`.
-    tsan: CC=cc -fsanitize=thread
-    asan: CC=cc -fsanitize=address
+    tsan: CC=cc -fsanitize=thread   -fno-omit-frame-pointer
+    asan: CC=cc -fsanitize=address  -fno-omit-frame-pointer
 
 # always compile pygolang from source and don't reuse binary pygolang wheels as
 # we compile each case with different CFLAGS.