runtime: implement weak.runtime_makeStrongFromWeak

[yohimik]

runtime: implement weak.runtime_makeStrongFromWeak

What this does

The runtime has weak.runtime_registerWeakPointer but not its counterpart, so a
program that reads a weak pointer back does not link. The missing function is 7
lines.

Weak pointers are not weak here. registerWeakPointer returns the pointer that
it got, so the value it refers to stays and the way back to a strong pointer is
the identity too. weak.Pointer.Value thus never reports a collected value,
which the documented contract permits.

Evidence

testdata/weak.go makes a weak pointer and reads it back. It is registered in
TestBuild under the minor >= 24 gate, because the weak package came with
Go 1.24.

Measured on macOS 26.6 arm64.

Build	Result
current dev	link error, weak/pointer.go:89: linker could not find symbol _weak.runtime_makeStrongFromWeak
with this change	weak value: 42

The program also builds for cortex-m-qemu, riscv-qemu and simavr.

A downstream product also ships binaries built with a fork that carries this
change, in the published release dispat v1.4.0.
https://github.com/yohimik/dispat/releases/tag/services%2Fdispat%2Fv1.4.0

Why it matters

crypto/tls uses weak.Pointer for its certificate cache, so this is a
prerequisite for the standard library crypto/tls on any target. Any user
program that uses weak hits the same link error today.

Scope

• One new function in src/runtime/runtime.go. No API change, no behaviour
  change for a program that does not use weak.

Related pull requests

This change is part of one body of work. Together the changes make programs that use the network and child processes work on hosted linux and macOS. A full CLI was tested end to end with all of them and ships binaries built this way, see dispat v1.4.0 in the evidence section.

In this repository

• sync: hand the RWMutex over instead of a re-test of the reader count #5630 sync, RWMutex hand-over. Bug fix, standalone.
• runtime: deliver os/signal notifications under the threads scheduler #5631 runtime, os/signal delivery with the threads scheduler. Bug fix, standalone.
• compiler,runtime: call fcntl through a C wrapper on darwin #5632 compiler and runtime, fcntl on darwin. Bug fix, standalone.
• runtime: implement weak.runtime_makeStrongFromWeak #5633 runtime, weak.runtime_makeStrongFromWeak. Needed by real crypto/tls.
• os: start processes with posix_spawn #5634 os, start processes with posix_spawn.
• loader: use the real crypto/tls on hosted linux and darwin #5635 loader, real crypto/tls on hosted linux and darwin.
• builder: declare the BSD socket API in the darwin libSystem stub #5636 builder, BSD socket symbols in the darwin libSystem stub.

In tinygo-org/net

• net/http: follow redirects in the client again net#72 net/http, follow redirects in the client again.
• net: report a name that does not exist as a not-found lookup error net#73 net, report a name that does not exist as a not-found error.
• net: extend the host netdev to darwin net#74 net, extend the host netdev to darwin.
• net/http: give the HTTPS client trust roots on darwin net#75 net/http, give the HTTPS client trust roots on darwin.

A merge order that works. The three bug fixes are independent. #5633 goes before #5635. HTTPS on linux needs only #5633 and #5635. Full darwin support also needs #5636, the net changes and a new src/net submodule pin.

[yohimik]

Rebased on dev after the 0.42.0 release. The change applies on top of v0.42.0
as released without a conflict, and the diff is unchanged.