arm64: pt_regs: remove stale big-endian layout

arm64: pt_regs: remove stale big-endian layout

For historical reasons the layout of struct pt_regs depends on the
configured endianness, with the order of the 'syscallno' and 'unused2'
fields varying dependent upon whether __AARCH64EB__ is defined. We no
longer depend on the order of these two fields and can remove the
ifdeffery.

The current conditional layout was introduced in commit:

  35d0e6fb4d219d64 ("arm64: syscallno is secretly an int, make it official")

At the time, this was necessary so that the entry assembly could use a
single STP instruction to save the pt_regs::{orig_x0,syscallno} fields,
without logic that was conditional on the endianness of the kernel:

| el0_svc_naked:
|         stp     x0, xscno, [sp, #S_ORIG_X0]     // save the original x0 and syscall number

This logic was converted to C in commit:

  f37099b6992a0b81 ("arm64: convert syscall trace logic to C")

Since that commit, we no longer manipulate pt_regs::orig_x0 from
assembly, and only manipulate pt_regs::syscallno as a 32-bit quantity
early in the kernel_entry assembly:

| /* Not in a syscall by default (el0_svc overwrites for real syscall) */
| .if     \el == 0
| mov     w21, #NO_SYSCALL
| str     w21, [sp, #S_SYSCALLNO]
| .endif

Given the above, there's no longer a need for the layout of
pt_regs::{syscallno,unused2} to depend on the endianness of the kernel.

This patch removes the ifdeffery and places 'syscallno' before 'unused2'
regardless of the endianess of the kernel. At the same time, 'unused2'
is renamed to 'unused', as it is the only unused field within pt_regs.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Puranjay Mohan <puranjay12@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Kalesh Singh <kaleshsingh@google.com>
Cc: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20241017092538.1859841-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>