internal/gocore: find new names of runtime._type and runtime.itab

This change makes (e.g.) `viewcore histogram` run without crashing cores
produced by current toolchains again. The output is still partially
wrong, in the sense that it outputs a mixture of named and unnamed
types:

    runtime.g size=448
    []*runtime.moduledata size=24
    [1+1?]*runtime.moduledata size=16
    [10+6?]uint8 size=16
    [1+3?]uint32 size=16

Still it's an improvement over the status quo.

In Go 1.21, runtime._type was converted from its own type into an alias
of internal/abi.Type. In the process, its fields became public too. For
an overview of the history:

 - https://go.dev/cl/462995 (internal/abi: refactor (basic) type struct into one definition)
 - https://go.dev/cl/484856 (internal/reflectlite, runtime: move more constants and types into internal/abi)
 - https://go.dev/cl/488435 (runtime: redefine _type to abi.Type; add rtype for methods)

In https://go.dev/cl/608475 (internal/gocore: support Go 1.22 allocation
headers), this was partially reflected in viewcore, but only just enough
to get basic tests for the current toolchain working.

There were still references to the old `runtime._type` and
`runtime.itab` (renamed `abi.ITab`), causing crashes for certain
operations, like `viewcore histogram` as well as the nascent dominator
tree code:

    panic: can't find type runtime._type [recovered]
            panic: can't find type runtime._type

    golang.org/x/debug/internal/gocore.(*Process).findType(...)
            /home/aktau/go/debug/internal/gocore/process.go:112
    golang.org/x/debug/internal/gocore.(*Process).runtimeType2Type(0xc00026a000, 0x46d8a0, 0x4ee3a8)
            /home/aktau/go/debug/internal/gocore/type.go:153 +0x14d9
    golang.org/x/debug/internal/gocore.(*Process).typeObject(0xc00026a000, 0x4ee3a0, 0xc000392460, {0x7b9e40, 0xc0001f4000}, 0xc00052d960)
            /home/aktau/go/debug/internal/gocore/type.go:636 +0xd45
    golang.org/x/debug/internal/gocore.(*Process).typeHeap.func1.2(0xc000582a08?)
            /home/aktau/go/debug/internal/gocore/type.go:514 +0x79
    golang.org/x/debug/internal/gocore.(*Process).ForEachRoot(0xc00026a000, 0xc00052db20)
            /home/aktau/go/debug/internal/gocore/object.go:220 +0x6a
    golang.org/x/debug/internal/gocore.(*Process).typeHeap.func1()
            /home/aktau/go/debug/internal/gocore/type.go:509 +0x179
    sync.(*Once).doSlow(0x6fe760?, 0xc00026a000?)
            /usr/lib/google-golang/src/sync/once.go:76 +0xb4
    sync.(*Once).Do(...)
            /usr/lib/google-golang/src/sync/once.go:67
    golang.org/x/debug/internal/gocore.(*Process).typeHeap(0x3?)
            /home/aktau/go/debug/internal/gocore/type.go:383 +0x3b
    golang.org/x/debug/internal/gocore.runLT(0xc00026a000)
            /home/aktau/go/debug/internal/gocore/dominator.go:89 +0x58
    golang.org/x/debug/internal/gocore.TestVersions.func2.1(0xc00012c680)
            /home/aktau/go/debug/internal/gocore/gocore_test.go:513 +0x5e

Change-Id: I8559c86f03d52b114f92756caf4487df75237983
Reviewed-on: https://go-review.googlesource.com/c/debug/+/634755
Auto-Submit: Nicolas Hillegeer <aktau@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Commit-Queue: Nicolas Hillegeer <aktau@google.com>