Fix DISABLE_PERL=on builds for non-assembly optimized platforms

[skmcgrail]

Description of changes:

I noticed that when attempting to build aws-lc with DISABLE_PERL=on the logic is not 100% accurate on whether it should expect to find optimized assembly for a given architecture or not. If you attempt to build aws-lc on an architecture with no provided assembly, and don't set DISABLE_PERL=off or set OPENSSL_NO_ASN=on the build may fail with the following message:

  Copying platform assembly files from /home/ec2-user/aws-lc-rs/aws-lc-sys/aws-lc/generated-src/linux-riscv64/crypto/ to /home/ec2-user/aws-lc-rs/target/debug/build/aws-lc-sys-6fc1a5d572af4537/out/build/aws-lc/crypto
  CMake Error at aws-lc/crypto/CMakeLists.txt:138 (file):
    file COPY cannot find
    "/home/ec2-user/aws-lc-rs/aws-lc-sys/aws-lc/generated-src/linux-riscv64/crypto/":
    No such file or directory.

However there is logic in this check that looks to see if $(ARCH) == generic, and will otherwise not set the ASSEMBLY_SOURCE variable. There are instances such as riscv64 where the processor is detected, and ARCH is purposefully not set to the value generic. This causes the build to fail in this specific configuration combination outlined above, even if we don't have assembly source for that platform purposefully. This adds a status message to indicate if a pre-generated assembly directory is non-existent, and allows the compilation to move forward. Note that BoringSSL does not have this issue as they don't provide pre-generated assembly files.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.