arch/arm64: Let the architecture select ARCH_HAVE_FORK.

Review of apache#19772 asked for this shape, and it applies to every architecture in
the series.

ARCH_HAVE_FORK described when it was available from inside its own definition,
which put the per-architecture condition somewhere nobody looks.  The
architecture now says so itself.

The condition repeats the ARCH_ADDRENV dependency rather than relying on it,
because a select bypasses depends on:  without that repetition an architecture
could offer fork() where there is no address environment to duplicate.

Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Author: casaroli

arch/Kconfig

@@ -26,6 +26,7 @@ config ARCH_ARM
 config ARCH_ARM64
 	bool "ARM64"
 	select ALARM_ARCH
+	select ARCH_HAVE_FORK if BUILD_KERNEL && ARCH_ADDRENV
 	select ARCH_64BIT
 	select ARCH_HAVE_BACKTRACE
 	select ARCH_HAVE_INTERRUPTSTACK
@@ -492,7 +493,6 @@ config ARCH_HAVE_VFORK
 
 config ARCH_HAVE_FORK
 	bool
-	default y if ARCH_ARM64 && ARCH_USE_MMU && !BUILD_PROTECTED
 	default n
 	depends on ARCH_ADDRENV
 	---help---