Add a lockdep build config and syscall-test recipes

[shailend-g]

Promote the lockdep go build tag to a bazel config, so any target can be
built with the runtime lock-order checker via --config=lockdep, mirroring
--config=race. The lockdep gotag is added to go_binary and go_test targets
when //tools:lockdep is set.

Also document in the Makefile how to run syscall tests with lockdep and
with the race detector, add a SENTRY_SIDECAR_TARGET knob so the race build
can replace the pure gvisor_sentry sidecar (the sentry runs as a separate
binary, so runsc-race alone only covers the CLI and gofer), and add a
make syscall-test-boot-log target to find the newest runsc boot log.

Demo usage. Each recipe assumes a deliberate fault injected into the
sentry to precipitate the expected failure.

1. Lockdep cycle (take t.mu then signalHandlers.mu in both orders under
   Getuid):

make syscall-tests TARGETS=//test/syscalls:uidgid_test_runsc_systrap_shared
BAZEL_OPTIONS=--config=lockdep OPTIONS=--nocache_test_results

The sentry panics; make syscall-test-boot-log TEST=uidgid names the log,
which shows the offending chain:

panic: WARNING: circular locking detected: kernel.taskMutex -> kernel.signalHandlersMutex
kernel.(*Task).LockdepDemoCycle pkg/sentry/kernel/threads.go
syscalls/linux.Getuid pkg/sentry/syscalls/linux/sys_identity.go
known lock chain: kernel.signalHandlersMutex -> kernel.taskMutex
kernel.(*TaskSet).newTask pkg/sentry/kernel/task_start.go

2. Data race in the sentry kernel (race two goroutines during boot):

make syscall-tests TARGETS=//test/syscalls:uname_test_runsc_systrap_shared
RUNSC_TARGET='--config=race //runsc:runsc-race'
SENTRY_SIDECAR_TARGET='--config=race //runsc/cmd/sentry:gvisor_sentry-race'
OPTIONS='--nocache_test_results --test_env=GORACE=halt_on_error=1'

make syscall-test-boot-log TEST=uname names the log; it holds the race:

WARNING: DATA RACE
Read at 0x... by goroutine N:
kernel.RaceDemo.func1 pkg/sentry/kernel/threads.go
Previous write at 0x... by goroutine M:
kernel.RaceDemo.func1 pkg/sentry/kernel/threads.go

3. Data race in runsc/boot (same race build, fault in the boot glue rather
   than the kernel package):

WARNING: DATA RACE
Read at 0x... by goroutine N:
runsc/boot.raceDemoBoot.func1 runsc/boot/loader.go
Previous write at 0x... by goroutine M:
runsc/boot.raceDemoBoot.func1 runsc/boot/loader.go

[EtiennePerot]

I'm just about to add per-platform sentries. We should not have this makefile assume that there is only one such binary.

Can you instead have the //:release target swap the runsc target it uses when --config=race?