builtins: switch to c11 from c99

This fixes an incorrect standard usage of GNU99 when the compiler check was for
the ISO standard C99.  Furthermore, bump the dependency up to C11.  The
motivation for this change is ARM EHABI compatibility with clang 3.8.  We rely
on a type definition redefinition which causes an error with -Werror builds.
This is problematic for FreeBSD builds.  Switching to C11 allows the
compatibility without the unnecessary pedantic warning.  The alternative would
be to clutter the support header with a `pragma clang diagnostic ignore`.  GCC
4.8+ and the supported clang revisions along with MSVC support enough of C11 to
allow building the builtins in C11 mode.  No functional change intended.

llvm-svn: 288099