linux-user/syscall.c: eliminate other explicit LFS usages

linux-user/syscall.c: eliminate other explicit LFS usages

Since we alwasy build with LFS enabled, and with -D_FILE_OFFSET_BITS=64
in particular, there is no need to use 64bit versions of various system
calls and constants, regular ones will do just fine.  Eliminate a few
last uses of the following constructs in linux-user/syscall.c:
  off64_t
  ftruncate64()
  lseek64()
  pread64()
  pwrite64()

This way it can be built on systems where the 64bit variants of
everything is not defined (since the system always uses 64bit
variants), such as on recent MUSL.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2215
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>