purgatory: Change default sha256 optimization to -O2

Change the default purgatory sha256 code optimization from -O0 to -O2, and add a
new arch specific makefile variable $(ARCH)_PURGATORY_SHA256_CFLAGS which can
over ride this default.  Set ia64_PURGATORY_SHA256_CFLAGS to -O0 to retain the
previous optimization level for ia64.

The purgatory sha256 code needs the be built with -O0 for the  ia64
architecture.  Currently this code is built with -O0 for all architectures,
which slows down the calculations for architectures which could otherwise
use -O2.

On arm64, it takes around 20 second to verify SHA in purgatory when
vmlinuz image is around 13MB and initramfs is around 30M with -O2
enabled. Otherwise, it takes more than 2 minutes.

Cc: Pratyush Anand <panand@redhat.com>
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Simon Horman <horms@verge.net.au>

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