Make Doom run by improving and fixing libc stubs, adding proper heap

allocation, adding a timer, a framebuffer swap, rectangle and buffer
draw syscall, moving the header files to the correct directory,
disabling stack protection, AVX and MMX, while enabling SSE, fix
interrupt handler popping in the wrong order, improve
load_segment_to_memory alignment, add load_from_ptr to framebuffer for
very quick drawing, serial_print as well as render font in
consolewriter, zero the memory in sbrk, add a fake file system and
methods to libxunil for DOOM, add correct printf, vsnprintf add _start
which calls the main function in libxunil, mark all registers as used
inside syscalls for no memory corruption, add build_helloworld.sh script

user/libxunil/include/stdlib.h

@@ -0,0 +1,24 @@
+#pragma once
+#include <inttypes.h>
+#include <stddef.h>
+
+void  *malloc(size_t size);
+void  *calloc(size_t nmemb, size_t size);
+void  *realloc(void *ptr, size_t size);
+void   free(void *ptr);
+
+void   exit(int status);
+void   abort(void);
+
+int    atoi(const char *s);
+long   atol(const char *s);
+double atof(const char *s);
+long   strtol(const char *s, char **endptr, int base);
+double strtod(const char *s, char **endptr);
+
+char  *getenv(const char *name);
+void   qsort(void *base, size_t nmemb, size_t size, int (*cmp)(const void *, const void *));
+int    abs(int x);
+int    system(const char *cmd);
+int    draw_pixel(uint32_t x, uint32_t y, uint32_t color);
+int    framebuffer_swap();