libc/unistd: Replace pipe() macro with proper function implementation

Replace the pipe() macro definition with a proper function implementation
to improve POSIX compliance and debugging capabilities. The new pipe()
function serves as a wrapper around pipe2() with flags set to 0.

Changes include:
- Convert pipe() from macro to function declaration in unistd.h
- Add lib_pipe.c implementation file with proper function documentation
- Update build system files (CMakeLists.txt and Make.defs) to include
  the new source file when CONFIG_PIPES is enabled
- Add pipe() entry to libc.csv for symbol tracking

This change allows for better debugging, proper symbol resolution,
and follows NuttX coding standards for library function implementations.

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

include/unistd.h

@@ -394,7 +394,7 @@ FAR void *sbrk(intptr_t incr);
 
 /* Special devices */
 
-#define pipe(fd) pipe2(fd, 0)
+int     pipe(int pipefd[2]);
 int     pipe2(int pipefd[2], int flags);
 
 /* Schedule an alarm */