coregrind/m_scheduler: Inform tools about thread ID while translating

This patch fixes the following assertion failure in DRD with
ENABLE_DRD_CONSISTENCY_CHECKS defined:

drd: drd_load_store.c:186 (vgDrd_trace_load): the 'impossible' happened.
==9543==    at 0x2803C4EC: report_and_quit (m_libcassert.c:279)
==9543==    by 0x2803C646: vgPlain_assert_fail (m_libcassert.c:359)
==9543==    by 0x2802A488: vgDrd_trace_load (drd_load_store.c:186)
==9543==    by 0x2805743E: vgPlain_translate (m_translate.c:1403)
==9543==    by 0x28088FCE: vgPlain_scheduler (scheduler.c:1014)
==9543==    by 0x280983D2: run_a_thread_NORETURN (syswrap-linux.c:103)


git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13862 a5019735-40e9-0310-863c-91ae7b9d1cf9