Use free instead of xfree to free altstack

	The altstack memory of a thread may be free'ed even after the VM is
	destructed. After that, GC is no longer available, so calling xfree
	may lead to a segfault.

	This changeset uses the bare free function to free the altstack memory
	instead of xfree. [Bug #18126]
	---
	 signal.c  | 5 ++++-
	 vm_core.h | 2 +-
	 2 files changed, 5 insertions(+), 2 deletions(-)