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>

libs/libc/libc.csv

@@ -192,6 +192,7 @@
 "ntohs","arpa/inet.h","","uint16_t","uint16_t"
 "opendir","dirent.h","","FAR DIR *","FAR const char *"
 "perror","stdio.h","defined(CONFIG_FILE_STREAM)","void","FAR const char *"
+"pipe","unistd.h","defined(CONFIG_PIPES) && CONFIG_DEV_PIPE_SIZE > 0","int","int [2]|FAR int *"
 "posix_fallocate","fcntl.h","","int","int","off_t","off_t"
 "posix_memalign","stdlib.h","","int","FAR void **","size_t","size_t"
 "preadv","sys/uio.h","","ssize_t","int","FAR const struct iovec *","int","off_t"