configure.ac/meson.build: Fix -latomic test

configure.ac/meson.build: Fix -latomic test

When compiling with LLVM 6.0 on x86 (32-bit) for Android, the test
fails to detect that -latomic is actually required, as the atomic
call is inlined.

In the code itself (src/util/disk_cache.c), we see this pattern:
p_atomic_add(cache->size, - (uint64_t)size);
where cache->size is an uint64_t *, and results in the following
link time error without -latomic:
src/util/disk_cache.c:628: error: undefined reference to '__atomic_fetch_add_8'

Fix the configure/meson test to replicate this pattern, which then
correctly realizes the need for -latomic.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>