ARM: 9444/1: add KEEP() keyword to ARM_VECTORS

Without this, the vectors are removed if LD_DEAD_CODE_DATA_ELIMINATION
is enabled.  At startup, the CPU (silently) hangs in the undefined
instruction exception as soon as the first timer interrupt arrives.

On my setup, the system also boots fine without the 2nd and 3rd KEEP()
statements, so I cannot tell whether these are actually required.

[nathan: Use OVERLAY_KEEP() to avoid breaking old ld.lld versions]

Cc: stable@vger.kernel.org
Fixes: ed0f9410 ("ARM: 9404/1: arm32: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION")
Signed-off-by: Christian Eggers <ceggers@arri.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>