Merge bitcoin/bitcoin#23947: build: use host_os instead of TARGET_OS in configure output

1bf3809 build: use host_os instead of TARGET_OS in configure output (fanquake)

Pull request description:

  `TARGET_OS` was convenient, as a readable host name for most of our
  targeted platforms, however unless we add more code to configure to
  detect more hosts, it's easier just use `host_os` (it's also more
  informative).

  i.e FreeBSD master
  ```bash
    target os       =
    build os        = freebsd13.0
  ```

  this PR:
  ```bash
    target os       = freebsd13.0
    build os        = freebsd13.0
  ```

ACKs for top commit:
  hebasto:
    ACK 1bf3809

Tree-SHA512: 606d92f60ce3f2f6ab1f54e29b5c179048c62ba51336b272c081b1e009128dd83705b181cfe30991c7a51d9c63e8ba2076bfed9e6112e7d1a74a7f947c5754f5

Author: fanquake

configure.ac

@@ -1939,7 +1939,7 @@ echo "  gprof enabled   = $enable_gprof"
 echo "  werror          = $enable_werror"
 echo "  LTO             = $enable_lto"
 echo
-echo "  target os       = $TARGET_OS"
+echo "  target os       = $host_os"
 echo "  build os        = $build_os"
 echo
 echo "  CC              = $CC"