Tweak gdb.trace/ftrace.exp for aarch64

Some tests are skipped on aarch64 unexpectedly because arg0exp isn't
set.  This patch is to set arg0exp to "$x0" for aarch64.

gdb/testsuite:

2015-12-15  Yao Qi  <yao.qi@linaro.org>

	* gdb.trace/ftrace.exp: Set arg0exp to "$x0" if target
	is aarch64*-*-*.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2015-12-15  Yao Qi  <yao.qi@linaro.org>
+
+	* gdb.trace/ftrace.exp: Set arg0exp to "$x0" if target
+	is aarch64*-*-*.
+
 2015-12-14  Sandra Loosemore  <sandra@codesourcery.com>
 
 	* gdb.base/history-duplicates.exp: Skip if no readline support.

gdb/testsuite/gdb.trace/ftrace.exp

@@ -243,6 +243,8 @@ if [is_amd64_regs_target] {
     set arg0exp "\$rdi"
 } elseif [is_x86_like_target] {
     set arg0exp "*(int *) (\$ebp + 8)"
+} elseif { [istarget "aarch64*-*-*"] } {
+    set arg0exp "\$x0"
 } else {
     set arg0exp ""
 }