patch 9.0.1388: Amiga: not all builds use gethostname()

Problem:    Amiga: not all builds use gethostname().
Solution:   Use gethostname() for all builds except AROS. (Ola Söder,
            closes #12107)
diff --git a/src/os_amiga.c b/src/os_amiga.c
index 0e0fa77..2d243b7 100644
--- a/src/os_amiga.c
+++ b/src/os_amiga.c
@@ -691,7 +691,7 @@
     void
 mch_get_host_name(char_u *s, int len)
 {
-#if defined(__amigaos4__) && defined(__CLIB2__)
+#if !defined(__AROS__)
     gethostname(s, len);
 #else
     vim_strncpy(s, "Amiga", len - 1);