cmd/link: add support for automatically generating .note.gnu.build-id

[segevfiner]

What version of Go are you using (go version)?

$ go version
go version go1.15 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build730694799=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I want the ability to automatically generate a .note.gnu.build-id section in Go binaries. The linker currently supports a flag to get one from the outside: -B, but this is supposed to be a CRC that is calculated by the linker rather than something that can reliably be set from the outside as a linker flag.

This is used by delve and other debuggers to find split debug symbols.

What did you expect to see?

I want a flag like the binutils linker has -Wl,-build-id that adds an automatically generated .note.gnu.build-id section.

What did you see instead?

There is only a flag to set it from the outside, but it is meant to be a CRC calculated by the linker to be used reliably to differentiate different debug symbols.