Revert "[X86][CET] Shadow stack fix for setjmp/longjmp"

This reverts commit 30962eca38ef02666ebcdded72a94f2cd0292d68.

This commit has been causing test asan failures on a build bot.

http://green.lab.llvm.org/green/job/clang-stage1-configure-RA/45108/

Original commit: https://reviews.llvm.org/D46181

llvm-svn: 331813