Add copy_file_range syscall stub

llvm/llvm-project#169405 has this:
```diff
--- a/libcxx/src/filesystem/operations.cpp
+++ b/libcxx/src/filesystem/operations.cpp
@@ -41,17 +41,10 @@
 #include <time.h>

 // since Linux 4.5 and FreeBSD 13, but the Linux libc wrapper is only provided by glibc >= 2.27 and musl
-#if defined(__linux__)
-#  if defined(_LIBCPP_GLIBC_PREREQ)
-#    if _LIBCPP_GLIBC_PREREQ(2, 27)
-#      define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
-#    endif
-#  elif _LIBCPP_HAS_MUSL_LIBC
-#    define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
-#  endif
-#elif defined(__FreeBSD__)
+#if _LIBCPP_GLIBC_PREREQ(2, 27) || _LIBCPP_HAS_MUSL_LIBC || defined(__FreeBSD__)
 #  define _LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE
 #endif
+
 #if __has_include(<sys/sendfile.h>)
 #  include <sys/sendfile.h>
 #  define _LIBCPP_FILESYSTEM_USE_SENDFILE
```

Before the PR, because we didn't define `__linux__`,
`_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE` was not defined. But after it
`#if defined(__linux__)` check was gone, and because we defined
`_LIBCPP_HAS_MUSL_LIBC`, `_LIBCPP_FILESYSTEM_USE_COPY_FILE_RANGE` was
defined as well.

After this PR, we started to have
`error: undefined symbol: copy_file_range` error on
`core*.test_dylink_exceptions_try_catch_6_*` tests.

---

This commit fixes the linker error by wiring up `SYS_copy_file_range`
and provides a stub implementation returning `-ENOSYS` for both legacy
JS syscalls and WASMFS. This allows `copy_file_range.c` from musl to be
compiled into the system library. The libcxx implementation falls back
to other mechanisms when receiving `ENOSYS`.

Author: aheejin

src/lib/libsigs.js

@@ -240,6 +240,7 @@ sigs = {
   __syscall_epoll_create1__sig: 'ii',
   __syscall_epoll_ctl__sig: 'iiiip',
   __syscall_epoll_pwait__sig: 'iipiipp',
+  __syscall_copy_file_range__sig: 'iipipii',
   __syscall_faccessat__sig: 'iipii',
   __syscall_fadvise64__sig: 'iijji',
   __syscall_fallocate__sig: 'iiijj',

src/lib/libsyscall.js

@@ -708,6 +708,8 @@ var SyscallsLibrary = {
   __syscall_epoll_ctl: (epfd, op, fd, ev) => -{{{ cDefs.ENOSYS }}},
   __syscall_epoll_pwait__nothrow: true,
   __syscall_epoll_pwait: (epfd, ev, maxevents, timeout, sigmask, sigsetsize) => -{{{ cDefs.ENOSYS }}},
+  __syscall_copy_file_range__nothrow: true,
+  __syscall_copy_file_range: (fd_in, off_in, fd_out, off_out, len, flags) => -{{{ cDefs.ENOSYS }}},
   __syscall_getcwd__deps: ['$lengthBytesUTF8', '$stringToUTF8'],
   __syscall_getcwd: (buf, size) => {
     if (size === 0) return -{{{ cDefs.EINVAL }}};

system/include/emscripten/syscalls.h

@@ -122,6 +122,7 @@ int __syscall_shutdown(int sockfd, int how, int unused1, int unused2, int unused
 int __syscall_epoll_create1(int flags);
 int __syscall_epoll_ctl(int epfd, int op, int fd, struct epoll_event *ev);
 int __syscall_epoll_pwait(int epfd, struct epoll_event *ev, int maxevents, int timeout, const sigset_t *sigmask, size_t sigsetsize);
+ssize_t __syscall_copy_file_range(int fd_in, off_t *off_in, int fd_out, off_t *off_out, size_t len, unsigned flags);
 
 #ifdef __cplusplus
 }

system/lib/libc/musl/arch/emscripten/bits/syscall.h

@@ -87,3 +87,4 @@
 #define SYS_epoll_create1	__syscall_epoll_create1
 #define SYS_epoll_ctl		__syscall_epoll_ctl
 #define SYS_epoll_pwait		__syscall_epoll_pwait
+#define SYS_copy_file_range	__syscall_copy_file_range

system/lib/wasmfs/syscalls.cpp

@@ -1887,4 +1887,13 @@ int __syscall_epoll_pwait(int epfd,
   return -ENOSYS;
 }
 
+ssize_t __syscall_copy_file_range(int fd_in,
+                                  off_t* off_in,
+                                  int fd_out,
+                                  off_t* off_out,
+                                  size_t len,
+                                  unsigned flags) {
+  return -ENOSYS;
+}
+
 } // extern "C"

tools/system_libs.py

@@ -1344,7 +1344,7 @@ def get_files(self):
 
     libc_files += files_in_path(
         path='system/lib/libc/musl/src/linux',
-        filenames=['getdents.c', 'gettid.c', 'utimes.c', 'statx.c', 'wait4.c', 'wait3.c', 'epoll.c'])
+        filenames=['getdents.c', 'gettid.c', 'utimes.c', 'statx.c', 'wait4.c', 'wait3.c', 'epoll.c', 'copy_file_range.c'])
 
     libc_files += files_in_path(
         path='system/lib/libc/musl/src/malloc',