linux-user/mmap build failure using Clang 10

When building with Clang 10 on Fedora 32, we get:

    CC      linux-user/mmap.o
  linux-user/mmap.c:720:49: error: result of comparison 'unsigned long' > 18446744073709551615 is always false [-Werror,-Wtautological-type-limit-compare]
          if ((unsigned long)host_addr + new_size > (abi_ulong)-1) {
              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~