Add endbr64/bti instruction at the start of the gadget, otherwise we'll
get a SIGILL when the gadget gets call. Fix the instruction that sets
the syscall number on arm64.

ok anton@, deraadt@

Fix the syscallwx target which is affected by both mimmutable(2) and
xonly by using a new gadget routine written in assembler with the sole
purpose of issuing a syscall. Since it needs to be copied t

Fix the syscallwx target which is affected by both mimmutable(2) and
xonly by using a new gadget routine written in assembler with the sole
purpose of issuing a syscall. Since it needs to be copied to wx memory,
place it in the rodata section.

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