Fix is_amd64_regs_target check in i386-avx512.exp

As reported by Jan here:

  https://sourceware.org/ml/gdb-patches/2018-09/msg00831.html

the check that sets the number of available registers is backwards.

gdb/testsuite/ChangeLog:

	* gdb.arch/i386-avx512.exp: Fix setting of nr_regs based on
	is_amd64_regs_target.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2018-10-01  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* gdb.arch/i386-avx512.exp: Fix setting of nr_regs based on
+	is_amd64_regs_target.
+
 2018-10-01  Alan Hayward  <alan.hayward@arm.com>
 
 	* gdb.arch/aarch64-sighandler-regs.c: New test.

gdb/testsuite/gdb.arch/i386-avx512.exp

@@ -93,9 +93,9 @@ gdb_test "break [gdb_get_line_number "third breakpoint here"]" \
 gdb_continue_to_breakpoint "continue to third breakpoint in main"
 
 if [is_amd64_regs_target] {
-    set nr_regs 8
-} else {
     set nr_regs 32
+} else {
+    set nr_regs 8
 }
 
 for { set r 0 } { $r < $nr_regs } { incr r } {