gnulib: Provide abort() implementation for gnulib

The recent gnulib updates require an implementation of abort(), but the
current macro provided by changeset:

  cd37d3d391 gnulib: Drop no-abort.patch

to config.h.in does not work with the clang compiler since it doesn't
provide a __builtin_trap() implementation, so this element of the
changeset needs to be reverted, and replaced.

After some discussion with Vladimir 'phcoder' Serbinenko and Daniel Kiper
it was suggested to bring back in the change from the changeset:

  db7337a3d3 * grub-core/gnulib/regcomp.c (regerror): ...

Which implements abort() as an inline call to grub_abort(), but since
that was made static by changeset:

  a8f15bceea * grub-core/kern/misc.c (grub_abort): Make static

it is also necessary to revert the specific part that makes it a static
function too.

Another implementation of abort() was found in grub-core/kern/compiler-rt.c
which needs to also be removed to be consistent.

Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/posix_wrap/stdlib.h

@@ -58,4 +58,10 @@ abs (int c)
   return (c >= 0) ? c : -c;
 }
 
+static inline void __attribute__ ((noreturn))
+abort (void)
+{
+  grub_abort ();
+}
+
 #endif