builtins: Fix the sysroot's reference to si_int in int_types.h

si_int is already defined in sysroot's siginfo.h
emutls.c includes pthread.h which includes asm/siginfo.h which
in turn includes asm-generic/siginfo.h and that defines si_int.
si_int is also defined in builtin's int_types.h and this leads to
errors. This patch fixes the issue by undefining the si_int in int_types.h

Differential Revision: http://reviews.llvm.org/D15086

llvm-svn: 254472