Object/COFF: ExportAddressTableEntry is a union of two RVAs.

The previous definition was wrong. See Microsoft PE/COFF specification
section 5.3.2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197204 91177308-0d34-0410-b5e6-96231b3b80d8
Change stack probing code for MingW.

Since gcc 4.6 the compiler uses ___chkstk_ms which has the same semantics as the
MS CRT function __chkstk. This simplifies the prologue generation a bit.

Reviewed by Rafael Espíndola. 

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197205 91177308-0d34-0410-b5e6-96231b3b80d8