Add click-buildtool compile-shortensyms.

This is better than putting && in a Linux kernel compile command,
apparently.
Fixincludes tweak for some Linux versions.
Linux compilation: Use debug_flags if it exists.

Which requires compiling a C file (conftest.c), since make mistakes
nonexistent.o for an asm file.

Centos/RHEL issues reported by Keith Schoenefeld.
CPUQueue: Too many CPUs is a runtime error, not a compile-time error.
Linuxmodule scheduler: Use newer CPU-getter interfaces.