Fix x86/x86_64 calling convention for _ExtInt

After speaking with Craig Topper about some recent defects, he pointed
out that _ExtInts should be passed indirectly if larger than the largest
int register, and like ints when smaller than that.  This patch
implements that.

Note that this changed the way vaargs worked quite a bit, but they still
work.

Differential Revision: https://reviews.llvm.org/D78785