builtins: Fix struct __emutls_control to match GCC

The type of size and align in struct __emutls_control must be
  typedef unsigned int gcc_word __attribute__((mode(word)));
to match GCC. When gcc_word is larger than size_t, which is the case
for x32, the upper extra bits are all zeros. We can use variables of
size_t to operate on size and align.

Fix one trivial C99 warning about mixed declaration and code.

Differential Revision: http://reviews.llvm.org/D16886

llvm-svn: 259824