Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239
Change-Id: I5b4123bc6709641315120a191e36cc57541349b2
diff --git a/libc/bionic/getauxval.cpp b/libc/bionic/getauxval.cpp
index 22922b9..607e89c 100644
--- a/libc/bionic/getauxval.cpp
+++ b/libc/bionic/getauxval.cpp
@@ -33,7 +33,7 @@
 #include <elf.h>
 #include <errno.h>
 
-__LIBC_HIDDEN__ ElfW(auxv_t)* __libc_auxv = NULL;
+__LIBC_HIDDEN__ ElfW(auxv_t)* __libc_auxv = nullptr;
 
 extern "C" unsigned long int getauxval(unsigned long int type) {
   for (ElfW(auxv_t)* v = __libc_auxv; v->a_type != AT_NULL; ++v) {