stub runtime_{Before,After}Exec for linkage

[archie2x]

Fixes:

u-root/cmds/core/init$ GOARCH={arm64,amd64} tinygo build -tags noasm
/usr/lib/go-1.22/src/syscall/exec_unix.go:282: linker could not find symbol syscall.runtime_BeforeExec
/usr/lib/go-1.22/src/syscall/exec_unix.go:308: linker could not find symbol syscall.runtime_AfterExec

I copied BigGo/src/runtime/proc.go for this, duplicating the //go:linkname

In BigGo, exec calls are serialized because of golang/go#19546 but uses a rwlock whose implementation would require a lot of file import from BigGo or a merge of tinygo/bigGo runtime against current policy. I may follow this up with a simple sync/Mutex.

@leongross ?

[leongross]

I think you are addressing an important issue here, that has not been taken care of atm.
I tried to implement sth similar bit quite did not get it working, although this would be a great benefit.

[archie2x]

I think you are addressing an important issue here, that has not been taken care of atm. I tried to implement sth similar bit quite did not get it working, although this would be a great benefit.

I saw your PR. I haven't pulled it / attempted to run it but I think it's missing these stubs for some reason? Does it work without them?

[leongross]

I checked out your changes and built the failing cmdlet switch_root with it and this works just fine.

For context: The previous errors were:

ld.lld: error: undefined symbol: syscall.runtime_BeforeExec
>>> referenced by exec_unix.go:279 (/usr/lib/golang/src/syscall/exec_unix.go:279)
>>>               /tmp/tinygo131981138/main.lto.main.o:(runtime.run$1$gowrapper)

ld.lld: error: undefined symbol: syscall.runtime_AfterExec
>>> referenced by exec_unix.go:305 (/usr/lib/golang/src/syscall/exec_unix.go:305)
>>>               /tmp/tinygo131981138/main.lto.main.o:(runtime.run$1$gowrapper)
failed to run tool: ld.lld
error: failed to link /tmp/tinygo131981138/main: exit status 1

[leongross]

Put this maybe before the function to generate the doc string?

[leongross]

nit:

Suggested change

	// Copyright 2014 The Go Authors. All rights reserved.
	// Copyright 2024 The Go Authors. All rights reserved.

[leongross]

@aykevl @deadprogram

[leongross]

On a kind of related note: #4384 implements the missing signals so the linker does not break the compilation

[deadprogram]

Thank you for the PR @archie2x and to @leongross for review. Now merging.

[leongross]

Thanks @deadprogram for merging.