Fil-C Linux/x86_64 version 0.681

This release includes bugfixes and a memory-safe ucontext implementation. Also included is some work in progress to support ARM64 (that work isn't done yet, so this release is still x86_64-only).

Changes:

• filcc --version prints a message that is less likely to confuse meson.
• ucontext APIs now work (see fil-c.org/context_switches).
• Fix a rare scavenger assert that happens if processes that use seccomp stall (for context switch reasons) for more than 300ms while setting up their filter.
• Make -static mean -static-pie so that the runtime's ioctl handling can assume that the low 4GB of the address space are not used.
• Fix ICE when compiling function calls that return unusual types.
• Lots of changes towards ARM64 support by @pcc. ARM64 support is not done (yet).

Guide to the available files:

• filc-0.681-linux-x86_64.tar.xz is the traditional, self-contained "pizfix" distribution. It only includes the compiler, doesn't require root, and is based on the musl libc. This is recommended for quickly trying out Fil-C.
• optfil-0.681-linux-x86_64.tar.xz is the /opt/fil distribution. It includes many useful programs like OpenSSH, sudo, tmux, and git. It's based on glibc 2.40, and requires root permissions. This is recommended for giving Fil-C deeper scrutiny.