perf probe: Fix dwarf regs table for x86_64

In 293d5b439483 ("perf probe: Support probing on offline cross-arch binary")
DWARF register tables were introduced for many architectures, with the one for
the "dx" register being broken for x86_64, which got noticed by the 'perf test
bpf' testcase, that has this difference from a successful run to one that
fails, with the aforementioned patch:

-Writing event: p:perf_bpf_probe/func _text+5197232 f_mode=+68(%di):x32 offset=%si:s64 orig=dx:s32
-Failed to write event: Invalid argument
-bpf_probe: failed to apply perf probe eventsFailed to add events selected by BPF
+Writing event: p:perf_bpf_probe/func _text+5197232 f_mode=+68(%di):x32 offset=%si:s64 orig=%dx:s32

Add the missing '%' to '%dx' to fix this.

Acked-by: Masami Hiramatsu <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Fixes: 293d5b439483 ("perf probe: Support probing on offline cross-arch binary")
Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/arch/x86/include/dwarf-regs-table.h

···
 };
 
 static const char * const x86_64_regstr_tbl[] = {
-	"%ax", "dx", "%cx", "%bx", "%si", "%di",
+	"%ax", "%dx", "%cx", "%bx", "%si", "%di",
 	"%bp", "%sp", "%r8", "%r9", "%r10", "%r11",
 	"%r12", "%r13", "%r14", "%r15",
 };