arm64: defconfig: Use DEBUG_INFO_REDUCED

We've had DEBUG_INFO enabled for arm64 defconfigs since the initial
commit.  This is probably not that frequently used but substantially
inflates the size of the build tree and amount of I/O needed during the
build.  This was causing issues with storage usage in some automated CI
environments which don't expect defconfigs to be quite this big, and
generally increases the resource consumption for both them and people
doing local builds.  The main use case for the debug info is decoding
things with scripts/faddr2line but that doesn't need the full
DEBUG_INFO, DEBUG_INFO_REDUCED is enough for it, so enable that by
default.

Without this patch my build tree is 6.8G, with it the size drops to 2G
(smaller than the 6.4G for allmodconfig!).

Suggested-by: Catalin Marinas <catalin.marinas@arm.com>
Reported-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210304174407.17537-1-broonie@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>