i386: Remove CET support

CET is only support for x86_64, this patch reverts:

  - faaee1f0 x86: Support shadow stack pointer in setjmp/longjmp.
  - be9ccd27 i386: Add _CET_ENDBR to indirect jump targets in
    add_n.S/sub_n.S
  - c02695d7 x86/CET: Update vfork to prevent child return
  - 5d844e1b i386: Enable CET support in ucontext functions
  - 124bcde6 x86: Add _CET_ENDBR to functions in crti.S
  - 562837c0 x86: Add _CET_ENDBR to functions in dl-tlsdesc.S
  - f753fa7d x86: Support IBT and SHSTK in Intel CET [BZ #21598]
  - 825b58f3 i386-mcount.S: Add _CET_ENDBR to _mcount and __fentry__
  - 7e119cd5 i386: Use _CET_NOTRACK in i686/memcmp.S
  - 177824e2 i386: Use _CET_NOTRACK in memcmp-sse4.S
  - 0a899af0 i386: Use _CET_NOTRACK in memcpy-ssse3-rep.S
  - 7fb61336 i386: Use _CET_NOTRACK in memcpy-ssse3.S
  - 77a8ae09 i386: Use _CET_NOTRACK in memset-sse2-rep.S
  - 00e7b76a i386: Use _CET_NOTRACK in memset-sse2.S
  - 90d15dc5 i386: Use _CET_NOTRACK in strcat-sse2.S
  - f1574581 i386: Use _CET_NOTRACK in strcpy-sse2.S
  - 4031d748 i386/sub_n.S: Add a missing _CET_ENDBR to indirect jump
  - target
  -
Checked on i686-linux-gnu.