Makefile: rm $(SOURCES), mark targets as PHONY
Makefile: rm $(SOURCES), mark targets as PHONY

Since go has its own way to track dependencies and rebuild if needed,
and it is efficient enough, let's drop using SOURCES variable, mark
all targets as PHONY and let golang do its job.

The primary motivation for this was concern about using find on every
make invocation to build the list of all sources.

Some unscientific performance analisys:

Before:
> $ time make
> make: 'runc' is up to date.
>
> real	0m0.202s
> user	0m0.177s
> sys	0m0.031s

After:
> $ time make
> go build -mod=vendor -buildmode=pie  -tags "seccomp selinux apparmor" -ldflags "-X main.gitCommit="5a8210a58bd0f07cc987e6201b4174e5b93fa115" -X main.version=1.0.0-rc10+dev " -o runc .
>
> real	0m0.149s
> user	0m0.315s
> sys	0m0.106s

So, it is slightly faster using the wall clock, uses more CPU, but
we can be sure the binary is always up to date.

This also fixes the Makefile to mark all targets as PHONY. The list
was generated by `grep -E '^[a-z-]+:' Makefile | sed 's/:.*//'`.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

dqminh/runc,mikebrow/runc,estesp/runc,opencontainers/runc,xiaochenshen/runc,opencontainers/runc,mikebrow/runc,rhatdan/runc,dqminh/runc,xiaochenshen/runc,rhatdan/runc,kolyshkin/runc,kolyshkin/runc,dqminh/runc,kolyshkin/runc,estesp/runc,mikebrow/runc,kolyshkin/runc,dqminh/runc,xiaochenshen/runc,estesp/runc,rhatdan/runc,opencontainers/runc,rhatdan/runc,opencontainers/runc