Add x86_64 compute levels as additional system types

When performing distributed builds of machine learning packages, it
would be nice if builders without the required SIMD instructions can
be excluded as build nodes.

Since x86_64 has accumulated a large number of different instruction
set extensions, listing all possible extensions would be unwieldy.
AMD, Intel, Red Hat, and SUSE have recently defined four different
microarchitecture levels that are now part of the x86-64 psABI
supplement and will be used in glibc 2.33:

https://gitlab.com/x86-psABIs/x86-64-ABI
https://lwn.net/Articles/844831/

This change uses libcpuid to detect CPU features and then uses them to
add the supported x86_64 levels to the additional system types. For
example on a Ryzen 3700X:

$ ~/aps/bin/nix -vv --version | grep "Additional system"
Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux

1 files changed, 7 insertions, 0 deletions

diff --git a/src/libutil/compute-levels.hh b/src/libutil/compute-levels.hh
new file mode 100644
index 000000000..8ded295f9
--- /dev/null
+++ b/src/libutil/compute-levels.hh
@@ -0,0 +1,7 @@
+#include "types.hh"
+
+namespace nix {
+
+StringSet computeLevels();
+
+}