libc/fopen: support fopen with mode 'm'

refs:
https://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
This is a fake implementation, in order for fopen to succeed,
Because the complete mmap mapping the entire file will waste a lot of
memory.

Signed-off-by: dongjiuzhu1 <dongjiuzhu1@xiaomi.com>

libs/libc/stdio/lib_fopen.c

@@ -165,7 +165,7 @@ int lib_mode2oflags(FAR const char *mode)
     {
       switch (*mode)
         {
-          /* Open for read access ("r{b|x|+}") */
+          /* Open for read access ("r{m|b|x|+}") */
 
           case 'r' :
             if (state == MODE_NONE)
@@ -264,6 +264,15 @@ int lib_mode2oflags(FAR const char *mode)
               }
             break;
 
+          /* Attempt to access the file using mmap. */
+
+          case 'm' :
+            if (state != MODE_R)
+              {
+                goto errout;
+              }
+            break;
+
           /* Open for binary access ("{r|w|a|x|+}b") */
 
           case 'b' :