seccomp: Generate the policy files at compile time

This change avoids having to run the genseccomp.py script every time a
policy file is edited, and instead generates these files at
compile-time.

Bug: None
Test: m
Test: find out/soong/ -name x86_64_global_policy.cpp  # Shows files
Test: generated policies are equivalent to original policies
Change-Id: I12461fe0c5fb02c008c1b2503fbb994b8aa2f56b