[SEH] Remove the old __C_specific_handler code now that WinEHPrepare works

This removes the -sehprepare flag and makes __C_specific_handler
functions always to use WinEHPrepare.

This was tested by building all of chromium_builder_tests and running a
few tests that use SEH, but if something breaks, we can revert this.

llvm-svn: 235557