Use -gcflags=--pgoprofile instead of -pgo when building stdlib (#4097)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Go 1.23 introduces `go tool preprofile` (ref:
https://github.com/golang/go/commit/e069587c8d530ec912614d138bbf59a1d1788fb9).
This tool allows folks to do the heavy lifting of processing their CPU
profile ahead of time, resulting in a pre-processed profile that leads
to quicker PGO builds later.

The `--pgo` flag to the `go` command only accepts raw profiles due to
https://github.com/golang/go/commit/081dc9fd8c9cf824f5caac3a03a1c8dfd86894c0
which makes the go command itself run the pre-processor on the input
profile, which leads to errors if the given profile is already
pre-processed. Since rules_go adds the `--pgo` flag when building the
stdlib, this leads to errors when passing a pre-processed profile via
`--@io_bazel_rules_go//go/config:pgoprofile`:

```
$ go version
go version go1.23.1 linux/amd64
$ go tool preprofile -i raw_profile -o preprocessed
$ bazel build --@io_bazel_rules_go//go/config:pgoprofile=$(pwd)/preprocessed .
...
preprofile: error parsing profile: error parsing profile: parsing profile: unknown wire type: 7
...
```

This PR uses the gcflag instead when building stdlib,
as that can accept either a raw or pre-processed profile. This is what
we already do within the compile action anyways:
https://github.com/bazelbuild/rules_go/blob/fb5f09191c968817d149ddf296c526289dd15219/go/tools/builders/compilepkg.go#L490-L492