crt: Provide onexit symbol alias

Now when there is working _onexit() function, provide also onexit() aliased
function, which already has declaration in stdlib.h header file.

Signed-off-by: Martin Storsjö <martin@martin.st>

mingw-w64-crt/def-include/crt-aliases.def.in

diff --git a/mingw-w64-crt/def-include/crt-aliases.def.in b/mingw-w64-crt/def-include/crt-aliases.def.in
index 4796549..9e7b13b 100644
--- a/mingw-w64-crt/def-include/crt-aliases.def.in
+++ b/mingw-w64-crt/def-include/crt-aliases.def.in
@@ -115,7 +115,7 @@
 ADD_UNDERSCORE(memicmp)
 ADD_UNDERSCORE(mkdir)
 ADD_UNDERSCORE(mktemp)
-; ADD_UNDERSCORE(onexit)
+; onexit function alias is provided by misc/_onexit.c
 ADD_UNDERSCORE(open)
 #ifndef CRTAPP
 ADD_UNDERSCORE(putch)

mingw-w64-crt/misc/_onexit.c

diff --git a/mingw-w64-crt/misc/_onexit.c b/mingw-w64-crt/misc/_onexit.c
index 9caf0f7..a51fa3f 100644
--- a/mingw-w64-crt/misc/_onexit.c
+++ b/mingw-w64-crt/misc/_onexit.c
@@ -11,3 +11,6 @@
   return atexit((void (__cdecl *)(void))func) == 0 ? func : NULL;
 }
 _onexit_t __cdecl (*__MINGW_IMP_SYMBOL(_onexit))(_onexit_t func) = _onexit;
+
+_onexit_t __attribute__ ((alias ("_onexit"))) __cdecl onexit(_onexit_t);
+extern _onexit_t (__cdecl * __attribute__ ((alias (__MINGW64_STRINGIFY(__MINGW_IMP_SYMBOL(_onexit))))) __MINGW_IMP_SYMBOL(onexit))(_onexit_t);