ARM: 9324/1: fix get_user() broken with veneer

ARM: 9324/1: fix get_user() broken with veneer

The 32-bit ARM kernel stops working if the kernel grows to the point
where veneers for __get_user_* are created.

AAPCS32 [1] states, "Register r12 (IP) may be used by a linker as a
scratch register between a routine and any subroutine it calls. It
can also be used within a routine to hold intermediate values between
subroutine calls."

However, bl instructions buried within the inline asm are unpredictable
for compilers; hence, "ip" must be added to the clobber list.

This becomes critical when veneers for __get_user_* are created because
veneers use the ip register since commit 02e541db0540 ("ARM: 8323/1:
force linker to use PIC veneers").

[1]: https://github.com/ARM-software/abi-aa/blob/2023Q1/aapcs32/aapcs32.rst

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>