Switch to using freestanding (as opoosed to hosted) compilation of
Halide runtime. This allows using clangs built-in stdint.h and
stdarg.h.

Switch variable argument calls to use va_ from stdarg.h rather than
__builtin_va_... per above change.

Switch all runtime uses of WEAK macro to get it from a new
runtime_internal.h file that includes stdint.h and stddef.h and
defines WEAK.

A few small formatting fixes.
Merge branch 'master' into clang_freestanding_runtime

Conflicts:
	src/runtime/mini_stdint.h