Fix Valgrind interceptor for nc_thread_starter. 
Fix Valgrind interceptor for nc_thread_starter. 

VG_GET_ORIG_FN must be called as close to the start of the interceptor as
possible. This change fixes memcheck failure at the first pthread_create with
--track-origins=yes. Newlib-only.

This is a retry of r5500.

BUG=none
TEST=./scons --mode=dbg-linux,nacl platform=x86-64 sdl=none buildbot=memcheck run_under_extra_args=--track-origins=yes run_thread_test
Review URL: http://codereview.chromium.org/7096010

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@5505 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client