gdb: Powerpc fix gdb.multi/multi-arch-exec.exp test

The expect file has a procedure append_arch_options which sets march based
the istarget.  The current if / else statement does not check for
powerpc64.  The else statement is hit which sets march to -m32.  This
results in compilation errors on 64-bit PowerPC.

This patch adds an if statement to check for powerpc64 and if true sets mach
to -m64.

The patch was tested on a Power 10 system.  No compile errors were generated.
The test completes with 1 expected pass and no failures.

gdb/testsuite/gdb.multi/multi-arch-exec.exp

@@ -71,7 +71,9 @@ proc append_arch2_options {options_var} {
 	}
     }
 
-    if [istarget "s390*-*-*"] {
+    if [istarget "powerpc64*-*-*"] {
+	set march "-m64"
+    } else if [istarget "s390*-*-*"] {
 	set march "-m31"
     } else {
 	set march "-m32"