Changes

Add Emscripten build support and JS callback logging backend

This change enables building libosmocore for sandboxed, non-POSIX
environments, specifically WebAssembly targets produced via the
Emscripten toolchain.

The broader motivation is to allow partial execution of selected
Osmocom components in isolated runtime environments where direct access
to hardware and traditional operating system facilities (filesystem,
sockets, privileged execution) is not available.

One intended use case is running a GSM 2G base station where the
radio-facing components are executed inside a web environment, while
the remaining Osmocom stack and core network components continue to run
unchanged on a conventional backend server. In this model, highly
stripped-down variants of osmo-bts and osmo-trx are built as static
WebAssembly libraries and executed in the browser, while depending on
core libraries such as libosmocore, libosmo-netif, and libosmo-abis.

A practical advantage of this approach is that no deployment or
privileged setup is required on the radio endpoint side. A user can
instantiate a radio endpoint with minimal configuration, while all
stateful logic and operational complexity remains centralized on the
backend. Multiple such radio endpoints may connect to the same backend
core network, effectively forming a single logical network from the
core network perspective, independent of the physical location of the
radio endpoints.

Existing libosmocore build logic and platform assumptions rely on the
availability of POSIX APIs and OS services which are not present in
WebAssembly runtimes. This currently prevents libosmocore from being
built for such targets without targeted, build-time adjustments. This
patch introduces the minimal set of changes required to enable such
builds, without affecting native platforms.

As part of this groundwork, a minimal callback-based logging hook is
introduced. When building for Emscripten, this hook allows forwarding
log messages to an external environment via a user-provided JavaScript
callback. This enables integration with browser-side logging or UI
infrastructure without introducing new logging backends or runtime
dependencies. For all other build targets, the hook resolves to a no-op
implementation and does not alter existing logging behavior.

No runtime behavior, protocol semantics, or network interactions are
changed by this patch. All modifications are strictly limited to
build-time and platform-specific code paths and are only active when
targeting Emscripten. Native builds and existing deployment scenarios
remain unaffected.

This patch is intended as groundwork. Follow-up changes, proposed
separately and incrementally, may extend similar support to other
Osmocom components such as libosmo-netif, libosmo-abis, osmo-bts, and
osmo-trx, while keeping all such changes optional and isolated from
native builds.

Change-Id: Ia8d5f4bb6570b5e055826f3a051e5e5896866e31