[libc] adjust linux's mman.h definitions (#79652)

Use definitions from `<linux/mman.h>` to dispatch arch-specific flag
values.
For example, `MCL_CURRENT/MCL_FUTURE/MCL_ONFAULT` are different on
different architectures.

Author: SchrodingerZhu

libc/include/llvm-libc-macros/linux/CMakeLists.txt

@@ -16,12 +16,6 @@ add_header(
     sys-ioctl-macros.h
 )
 
-add_header(
-  sys_mman_macros
-  HDR
-    sys-mman-macros.h
-)
-
 add_header(
   sys_random_macros
   HDR

libc/include/llvm-libc-macros/linux/sys-mman-macros.h

@@ -9,8 +9,40 @@
 #ifndef __LLVM_LIBC_MACROS_SYS_MMAN_MACROS_H
 #define __LLVM_LIBC_MACROS_SYS_MMAN_MACROS_H
 
-#ifdef __linux__
-#include "linux/sys-mman-macros.h"
+// Use definitions from <linux/mman.h> to dispatch arch-specific flag values.
+// For example, MCL_CURRENT/MCL_FUTURE/MCL_ONFAULT are different on different
+// architectures.
+#if __has_include(<linux/mman.h>)
+#include <linux/mman.h>
+#else
+#error "cannot use <sys/mman.h> without proper system headers."
+#endif
+
+// Some posix standard flags may not be defined in system headers.
+// Posix mmap flags.
+#ifndef MAP_FAILED
+#define MAP_FAILED ((void *)-1)
+#endif
+
+// Posix memory advise flags. (posix_madvise)
+#ifndef POSIX_MADV_NORMAL
+#define POSIX_MADV_NORMAL MADV_NORMAL
+#endif
+
+#ifndef POSIX_MADV_SEQUENTIAL
+#define POSIX_MADV_SEQUENTIAL MADV_SEQUENTIAL
+#endif
+
+#ifndef POSIX_MADV_RANDOM
+#define POSIX_MADV_RANDOM MADV_RANDOM
+#endif
+
+#ifndef POSIX_MADV_WILLNEED
+#define POSIX_MADV_WILLNEED MADV_WILLNEED
+#endif
+
+#ifndef POSIX_MADV_DONTNEED
+#define POSIX_MADV_DONTNEED MADV_DONTNEED
 #endif
 
 #endif // __LLVM_LIBC_MACROS_SYS_MMAN_MACROS_H