Spec conformance

This branch of runc implements the OCI Runtime Spec v1.2.1 for the linux platform.

The following features are not implemented yet:

Spec version	Feature	PR
v1.1.0	`SECCOMP_FILTER_FLAG_WAIT_KILLABLE_RECV`	#3862

Architectures

The following architectures are supported:

runc binary	seccomp
`amd64`	`SCMP_ARCH_X86`, `SCMP_ARCH_X86_64`, `SCMP_ARCH_X32`
`arm64`	`SCMP_ARCH_ARM`, `SCMP_ARCH_AARCH64`
`armel`	`SCMP_ARCH_ARM`
`armhf`	`SCMP_ARCH_ARM`
`ppc64le`	`SCMP_ARCH_PPC64LE`
`riscv64`	`SCMP_ARCH_RISCV64`
`s390x`	`SCMP_ARCH_S390`, `SCMP_ARCH_S390X`

The runc binary might be compilable for i386, big-endian PPC64, and several MIPS variants too, but these architectures are not officially supported.