Silence most -Wzero-as-null-pointer-constant diagnostics

Replace 0 by NULL and {0} by {}.

Omit a few cases that aren't so trivial to fix.

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>
Link: <https://software.codidact.com/posts/292718/292759#answer-292759>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

sysdeps/unix/sysv/linux/brk.c

@@ -22,7 +22,7 @@
 #include <brk_call.h>
 
 /* This must be initialized data because commons can't have aliases.  */
-void *__curbrk = 0;
+void *__curbrk = NULL;
 
 #if HAVE_INTERNAL_BRK_ADDR_SYMBOL
 /* Old braindamage in GCC's crtstuff.c requires this symbol in an attempt