[AArch64] Add __builtin_sponentry, for calling setjmp in MinGW

In MinGW, setjmp isn't expanded as a builtin in the compiler (like it
is for MSVC), but manually hooked up as calls to the right underlying
functions in headers. Using the actual CRT's real setjmp/longjmp
functions requires this intrinsic. (Currently this is worked around by
using MinGW specific reimplementations of setjmp/longjmp on aarch64.)

Differential Revision: https://reviews.llvm.org/D61592

llvm-svn: 360082