crypto: arm - workaround for building with old binutils

Old versions of binutils (before 2.23) do not yet understand the
crypto-neon-fp-armv8 fpu instructions, and an attempt to build these
files results in a build failure:

arch/arm/crypto/aes-ce-core.S:133: Error: selected processor does not support ARM mode `vld1.8 {q10-q11},[ip]!'
arch/arm/crypto/aes-ce-core.S:133: Error: bad instruction `aese.8 q0,q8'
arch/arm/crypto/aes-ce-core.S:133: Error: bad instruction `aesmc.8 q0,q0'
arch/arm/crypto/aes-ce-core.S:133: Error: bad instruction `aese.8 q0,q9'
arch/arm/crypto/aes-ce-core.S:133: Error: bad instruction `aesmc.8 q0,q0'

Since the affected versions are still in widespread use, and this breaks
'allmodconfig' builds, we should try to at least get a successful kernel
build. Unfortunately, I could not come up with a way to make the Kconfig
symbol depend on the binutils version, which would be the nicest solution.

Instead, this patch uses the 'as-instr' Kbuild macro to find out whether
the support is present in the assembler, and otherwise emits a non-fatal
warning indicating which selected modules could not be built.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: http://storage.kernelci.org/next/next-20150410/arm-allmodconfig/build.log
Fixes: 864cbeed

 ("crypto: arm - add support for SHA1 using ARMv8 Crypto Instructions")
[ard.biesheuvel:
 - omit modules entirely instead of building empty ones if binutils is too old
 - update commit log accordingly]
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>