add powerpc64 and s390x to list of supported archs in INSTALL file

INSTALL

@@ -69,12 +69,20 @@ and ABI combinations:
       that does not use FPU registers or instructions is available
 
 * PowerPC
-    * Only 32-bit is supported
     * Compiler toolchain must provide 64-bit long double, not IBM
       double-double or IEEE quad
     * For dynamic linking, compiler toolchain must be configured for
       "secure PLT" variant
 
+* PowerPC64
+    * Both little and big endian variants are supported
+    * Compiler toolchain must provide 64-bit long double, not IBM
+      double-double or IEEE quad
+    * Compiler toolchain must use the new (ELFv2) ABI regardless of
+      whether it is for little or big endian
+
+* S390X (64-bit S390)
+
 * SuperH (SH)
     * Standard ELF ABI or FDPIC ABI (shared-text without MMU)
     * Little-endian by default; big-engian variant also supported