x86: Fix large model calls to __chkstk for dynamic allocas

x86: Fix large model calls to __chkstk for dynamic allocas

In the large code model, we now put __chkstk in %r11 before calling it.

Refactor the code so that we only do this once. Simplify things by using
__chkstk_ms instead of __chkstk on cygming. We already use that symbol
in the prolog emission, and it simplifies our logic.

Second half of PR18582.

llvm-svn: 227519