Revert "crypto: arm64/sha - Add constant operand modifier to ASM_EXPORT"

This reverts commit 42ae2922.  It
causes a regression with older versions of gcc.  The consensus is
that this should instead be fixed in clang.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>