tcg/s390x: Rename from tcg/s390

This emphasizes that we don't support s390, only 64-bit s390x hosts.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

meson.build

@@ -268,8 +268,6 @@ if not get_option('tcg').disabled()
     tcg_arch = 'tci'
   elif config_host['ARCH'] == 'sparc64'
     tcg_arch = 'sparc'
-  elif config_host['ARCH'] == 's390x'
-    tcg_arch = 's390'
   elif config_host['ARCH'] in ['x86_64', 'x32']
     tcg_arch = 'i386'
   elif config_host['ARCH'] == 'ppc64'