games/zdoom: fix the build with llvm16 in base

The 'register' keyword now raises an error, so patch out those parts.
Patch generally taken from gzdoom commit 3c044ebd5eae343.  No functional
change as a result of this.

games/zdoom/files/patch-src_am__map.cpp

@@ -0,0 +1,15 @@
+--- src/am_map.cpp.orig	2016-02-22 02:11:13 UTC
++++ src/am_map.cpp
+@@ -1692,9 +1692,9 @@ bool AM_clipMline (mline_t *ml, fline_t *fl)
+ 		TOP		=8
+ 	};
+
+-	register int outcode1 = 0;
+-	register int outcode2 = 0;
+-	register int outside;
++	int outcode1 = 0;
++	int outcode2 = 0;
++	int outside;
+
+ 	fpoint_t tmp = { 0, 0 };
+ 	int dx;

games/zdoom/files/patch-src_md5.cpp

@@ -0,0 +1,11 @@
+--- src/md5.cpp.orig	2016-02-22 02:11:13 UTC
++++ src/md5.cpp
+@@ -166,7 +166,7 @@ MD5Transform(DWORD buf[4], const DWORD in[16])
+ void
+ MD5Transform(DWORD buf[4], const DWORD in[16])
+ {
+-	register DWORD a, b, c, d;
++	DWORD a, b, c, d;
+
+ 	a = buf[0];
+ 	b = buf[1];

games/zdoom/files/patch-src_p__spec.cpp

@@ -9,3 +9,21 @@
  			{
  				{  0,  1 }, {  0,  2 }, {  0,  4 },
  				{ -1,  0 }, { -2,  0 }, { -4,  0 },
+@@ -1826,7 +1826,7 @@ static void P_SpawnScrollers(void)
+
+ 		switch (special)
+ 		{
+-			register int s;
++			int s;
+
+ 		case Scroll_Ceiling:
+ 		{
+@@ -2357,7 +2357,7 @@ static void P_SpawnPushers ()
+ {
+ 	int i;
+ 	line_t *l = lines;
+-	register int s;
++	int s;
+
+ 	for (i = 0; i < numlines; i++, l++)
+ 	{