Interestingly, everything points to something special in the glibc version of the buildbot. Usually the end of the stack for threads is:

(gdb)
#22 0x00007ffff7f82a9d in start_thread () from /usr/lib/libpthread.so.0
(gdb)
#23 0x00007ffff7d23a43 in clone () from /usr/lib/libc.so.6

but in this buildbot we get:

#24 0x000003fffde88b22 in start_thread () from /lib64/libpthread.so.0
(gdb)
#25 0x000003fffdb7164a in thread_start () from /lib64/libc.so.6
(gdb)
PC not saved

I think that the combination of glibc + gdb is producing this problem probably because the optimization level of glibc.