PR217695 malloc/calloc/realloc/memalign failure doesn't set errno to ENOMEM

PR217695 malloc/calloc/realloc/memalign failure doesn't set errno to ENOMEM

When one of the allocation functions in vg_replace_malloc failed
they return NULL, but didn't set errno. This is slightly tricky since
errno is implementation defined and might be a macro. In the case of
glibc ernno is defined as:

  extern int *__errno_location (void) __THROW __attribute__ ((__const__));
  #define errno (*__errno_location ())

We can use the same trick as we use for __libc_freeres in
coregrind/vg_preloaded.c. Define the function as "weak". This means
it will only be defined if another library (glibc in this case)
actually provides a definition. Otherwise it will be NULL.
So we will only call it if it is defined and one of the allocation
functions failed, returned NULL.

Include a new linux only memcheck testcase, enomem.vgtest.

https://bugs.kde.org/show_bug.cgi?id=217695