arch/arm: Restore vfork() on 32-bit ARM kernel builds.

ARCH_ARM has selected ARCH_HAVE_VFORK only "if !BUILD_KERNEL" since the
fork()/vfork() split.  That condition was deliberate but temporary:  it was
added because the fork family had never worked on a 32-bit ARM kernel build --
the entry point in fork.S snapshots the kernel-side stub rather than the
caller, so a child resumes at a kernel address -- and said in as many words
that "arch/arm takes the condition off again in the patch that adds its
saved-syscall-frame path".

That patch is the one before this.  arm_syscall() records the caller's
exception frame in xcp.sregs and arm_fork() builds the child from it, so the
condition has nothing left to protect against.

Cortex-M is unaffected either way -- BUILD_KERNEL depends on ARCH_USE_MMU,
which it does not have -- so the only configurations this changes are the
MMU-capable ARM ports, which are exactly the ones the previous commit fixed.

Verified on qemu-armv7a:knsh under qemu-system-arm:  ostest's vfork_test
passes, where before the change vfork() was absent.

Assisted-by: Claude Code:claude-opus-5
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

arch/Kconfig

@@ -11,7 +11,7 @@ config ARCH_ARM
 	bool "ARM"
 	select ARCH_HAVE_BACKTRACE
 	select ARCH_HAVE_INTERRUPTSTACK
-	select ARCH_HAVE_VFORK if !BUILD_KERNEL
+	select ARCH_HAVE_VFORK
 	select ARCH_HAVE_STACKCHECK
 	select ARCH_HAVE_CUSTOMOPT
 	select ARCH_HAVE_STDARG_H