rseq/selftests: x86: use ud1 instruction as RSEQ_SIG opcode

rseq/selftests: x86: use ud1 instruction as RSEQ_SIG opcode

Use ud1 as the guard instruction for the restartable sequence abort
handler. Its benefit compared to nopl is to trap execution if the
program ends up trying to execute it by mistake, which makes debugging
easier.

The 4-byte signature per se is unchanged (it is the instruction
operand). Only the opcode is changed from nopl to ud1.

commit 24fa5d1efe98 "rseq/selftests: x86: use ud1 instruction as RSEQ_SIG
opcode" in the Linux kernel.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>