Detect appropriate HWAES flags for gcc builds

[amrox]

Fixes: #365

If the compiler was detected as gcc, the CMake scripts would
automatically apply the hardware acceleration flags for x86_64. This
merely checks the CMAKE_SYSTEM_PROCESSOR variable and applies the
appropriate flags for the target system.

[asoffer]

Thanks for this, looks good! We have a change that landed internally a few days ago and will be pushed with our next release. I believe that what you have here will be an added improvement on top of that, so once that gets pushed to GitHub, I'll pull this change in as well.

[lilianmoraru]

For some reason, "CMAKE_SYSTEM_PROCESSOR" for me is always unset(x86_64 or cross-compiling to ARM with a Toolchain file).

[amrox]

@lilianmoraru Interesting. It seems to be set when I compile natively on macOS, and it's also set by the Emscripten and arm64 toolchains I use. What toolchain are you using for cross compiling?

[amrox]

@asoffer Happy to help, and open to feedback. I also noticed that the ABSL_RANDOM_HWAES_X64_FLAGS is set explicitly here too

abseil-cpp/absl/copts/AbseilConfigureCopts.cmake

Line 23 in 19b021c

set(ABSL_RANDOM_RANDEN_COPTS "${ABSL_RANDOM_HWAES_X64_FLAGS}")

so that may need to be updated.

[amrox]

It looks like a more robust fixed landed in commit 078b89b#diff-34f9b8e4d402381f2a234c311b7fbca6R8

Its working for me, so I'm going to go ahead and close this PR.