Linux: Remove rseq support

The kernel ABI is not finalized, and there are now various proposals
to change the size of struct rseq, which would make the glibc ABI
dependent on the version of the kernels used for building glibc.
This is of course not acceptable.

This reverts commit 48699da1c4 ("elf:
Support at least 32-byte alignment in static dlopen"), commit
8f4632deb3 ("Linux: rseq registration
tests"), commit 6e29cb3f61 ("Linux: Use
rseq in sched_getcpu if available"), and commit
0c76fc3c2b ("Linux: Perform rseq
registration at C startup and thread creation"), resolving the conflicts
introduced by the ARC port and the TLS static surplus changes.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

sysdeps/unix/sysv/linux/arm/bits/rseq.h

@@ -1,83 +0,0 @@
-/* Restartable Sequences Linux arm architecture header.
-   Copyright (C) 2020 Free Software Foundation, Inc.
-
-   The GNU C Library is free software; you can redistribute it and/or
-   modify it under the terms of the GNU Lesser General Public
-   License as published by the Free Software Foundation; either
-   version 2.1 of the License, or (at your option) any later version.
-
-   The GNU C Library is distributed in the hope that it will be useful,
-   but WITHOUT ANY WARRANTY; without even the implied warranty of
-   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
-   Lesser General Public License for more details.
-
-   You should have received a copy of the GNU Lesser General Public
-   License along with the GNU C Library; if not, see
-   <https://www.gnu.org/licenses/>.  */
-
-#ifndef _SYS_RSEQ_H
-# error "Never use <bits/rseq.h> directly; include <sys/rseq.h> instead."
-#endif
-
-/*
-   RSEQ_SIG is a signature required before each abort handler code.
-
-   It is a 32-bit value that maps to actual architecture code compiled
-   into applications and libraries.  It needs to be defined for each
-   architecture.  When choosing this value, it needs to be taken into
-   account that generating invalid instructions may have ill effects on
-   tools like objdump, and may also have impact on the CPU speculative
-   execution efficiency in some cases.
-
-   - ARM little endian
-
-   RSEQ_SIG uses the udf A32 instruction with an uncommon immediate operand
-   value 0x5de3.  This traps if user-space reaches this instruction by mistake,
-   and the uncommon operand ensures the kernel does not move the instruction
-   pointer to attacker-controlled code on rseq abort.
-
-   The instruction pattern in the A32 instruction set is:
-
-   e7f5def3    udf    #24035    ; 0x5de3
-
-   This translates to the following instruction pattern in the T16 instruction
-   set:
-
-   little endian:
-   def3        udf    #243      ; 0xf3
-   e7f5        b.n    <7f5>
-
-   - ARMv6+ big endian (BE8):
-
-   ARMv6+ -mbig-endian generates mixed endianness code vs data: little-endian
-   code and big-endian data.  The data value of the signature needs to have its
-   byte order reversed to generate the trap instruction:
-
-   Data: 0xf3def5e7
-
-   Translates to this A32 instruction pattern:
-
-   e7f5def3    udf    #24035    ; 0x5de3
-
-   Translates to this T16 instruction pattern:
-
-   def3        udf    #243      ; 0xf3
-   e7f5        b.n    <7f5>
-
-   - Prior to ARMv6 big endian (BE32):
-
-   Prior to ARMv6, -mbig-endian generates big-endian code and data
-   (which match), so the endianness of the data representation of the
-   signature should not be reversed.  However, the choice between BE32
-   and BE8 is done by the linker, so we cannot know whether code and
-   data endianness will be mixed before the linker is invoked.  So rather
-   than try to play tricks with the linker, the rseq signature is simply
-   data (not a trap instruction) prior to ARMv6 on big endian.  This is
-   why the signature is expressed as data (.word) rather than as
-   instruction (.inst) in assembler.  */
-
-#ifdef __ARMEB__
-# define RSEQ_SIG    0xf3def5e7      /* udf    #24035    ; 0x5de3 (ARMv6+) */
-#else
-# define RSEQ_SIG    0xe7f5def3      /* udf    #24035    ; 0x5de3 */
-#endif