Add PowerPC support to gdb.cp/call-method-register.cc

This patch adds the needed define ASM_REG for PowerPC.

The patch was run on a Power 10 system.  The gdb Summary for the run lists
2 expected passes, no unexpected failures or untested testcases.

Please let me know if this patch is acceptable for mainline.

                         Carl Love

gdb/testsuite/gdb.cp/call-method-register.cc

diff --git a/gdb/testsuite/gdb.cp/call-method-register.cc b/gdb/testsuite/gdb.cp/call-method-register.cc
index 06e9e53..d60fee0 100644
--- a/gdb/testsuite/gdb.cp/call-method-register.cc
+++ b/gdb/testsuite/gdb.cp/call-method-register.cc
@@ -19,6 +19,8 @@
 # define ASM_REG "rax"
 #elif defined __i386__
 # define ASM_REG "eax"
+#elif defined __powerpc64__
+# define ASM_REG "r9"
 #else
 # error "port me"
 #endif