[gdb/testsuite] Add xfail for failure to break on ppc printf

With test-case gdb.server/sysroot.exp, we run into:
...
(gdb) break printf^M
Breakpoint 2 at 0x7ffff7c2045c^M
(gdb) continue^M
Continuing.^M
[Inferior 1 (process 469972) exited normally]^M
(gdb) FAIL: $exp: sysroot=local: continue to printf (the program exited)
...

The problem is that while breakpoint is indeed set on printf:
...
(gdb) b printf
Breakpoint 2 at 0x7ffff7c2045c
(gdb) disassemble 0x7ffff7c2045c
Dump of assembler code for function printf@@GLIBC_2.17:
...
the function that is called instead is __printfieee128.

Glibc commit f05ab7c4a9 ("ldbl-128ibm-compat: Add local aliases for printf
family symbols") should fix this, but that's available starting version 2.43.

Add an xfail for older glibc (without backport of that patch).

Tested on ppc64le-linux and x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29989