[lldb] Don't use ::fork or ::vfork on watchOS or tvOS

Update lldb-server to not use fork or vfork on watchOS and tvOS as these
functions are explicitly marked unavailable there.

llvm-project/lldb/test/API/tools/lldb-server/main.cpp:304:11:
error: 'fork' is unavailable: not available on watchOS
      if (fork() == 0)
          ^
WatchSimulator6.2.sdk/usr/include/unistd.h:447:8: note: 'fork' has been
explicitly marked unavailable here
pid_t    fork(void) __WATCHOS_PROHIBITED __TVOS_PROHIBITED;
         ^
llvm-project/lldb/test/API/tools/lldb-server/main.cpp:307:11:
error: 'vfork' is unavailable: not available on watchOS
      if (vfork() == 0)
          ^
WatchSimulator6.2.sdk/usr/include/unistd.h:602:8: note: 'vfork' has been
explicitly marked unavailable here
pid_t    vfork(void) __WATCHOS_PROHIBITED __TVOS_PROHIBITED;
         ^