[gdb/testsuite] Fix printf regexp in gdb.server/sysroot.exp

When running gdb.server/sysroot.exp, I run into this FAIL:
...
(gdb) continue^M
Continuing.^M
^M
Breakpoint 2, __printf (format=0x4005c4 "Hello World!\n") at printf.c:28^M
28      {^M
(gdb) FAIL: gdb.server/sysroot.exp: sysroot=local: continue to printf
...
for this test:
...
    gdb_test "continue" "Breakpoint $decimal.* printf .*" "continue to printf"
...

Without debug info for glibc installed, we have instead:
...
(gdb) continue^M
Continuing.^M
^M
Breakpoint 2, 0x00007ffff773c550 in printf () from /lib64/libc.so.6^M
(gdb) PASS: gdb.server/sysroot.exp: sysroot=local: continue to printf
...

Fix this by allowing for GLIBC's printf alias __printf to be printed:
...
    gdb_test "continue" "Breakpoint $decimal.* (__)?printf .*" \
      "continue to printf"
...

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-11  Tom de Vries  <tdevries@suse.de>

	* gdb.server/sysroot.exp: Allow GLIBC's printf alias __printf.