ARM: 9448/1: Use an absolute path to unified.h in KBUILD_AFLAGS

ARM: 9448/1: Use an absolute path to unified.h in KBUILD_AFLAGS

[ Upstream commit 87c4e1459e80bf65066f864c762ef4dc932fad4b ]

After commit d5c8d6e0fa61 ("kbuild: Update assembler calls to use proper
flags and language target"), which updated as-instr to use the
'assembler-with-cpp' language option, the Kbuild version of as-instr
always fails internally for arch/arm with

  <command-line>: fatal error: asm/unified.h: No such file or directory
  compilation terminated.

because '-include' flags are now taken into account by the compiler
driver and as-instr does not have '$(LINUXINCLUDE)', so unified.h is not
found.

This went unnoticed at the time of the Kbuild change because the last
use of as-instr in Kbuild that arch/arm could reach was removed in 5.7
by commit 541ad0150ca4 ("arm: Remove 32bit KVM host support") but a
stable backport of the Kbuild change to before that point exposed this
potential issue if one were to be reintroduced.

Follow the general pattern of '-include' paths throughout the tree and
make unified.h absolute using '$(srctree)' to ensure KBUILD_AFLAGS can
be used independently.

Closes: https://lore.kernel.org/CACo-S-1qbCX4WAVFA63dWfHtrRHZBTyyr2js8Lx=Az03XHTTHg@mail.gmail.com/
Cc: stable@vger.kernel.org
Fixes: d5c8d6e0fa61 ("kbuild: Update assembler calls to use proper flags and language target")
Reported-by: KernelCI bot <bot@kernelci.org>
Reviewed-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
[ No KBUILD_RUSTFLAGS in <=6.12 ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>