X86: repair export compatibility with MinGW/cygwin

X86: repair export compatibility with MinGW/cygwin

Both MinGW and cygwin (i686) construct export directives without the global
leader prefix.  This is mostly due to the fact that they use GNU ld which does
not correctly handle the export directive.  This apparently has been been broken
for a while.  However, this was recently reported as being broken by
mingwandroid and diorcety of the msys2 project.

Remove the global leader prefix if targeting MinGW or cygwin, otherwise, retain
the global leader prefix.  Add an explicit test for cygwin's behaviour of export
directives.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207926 91177308-0d34-0410-b5e6-96231b3b80d8