Undefine FORTIFY_SOURCE for longjmp

See the comment in the source file, we can't have FORTIFY_SOURCE enabled for longjmp code, so undefine it.

See the source for longjmp_chk in glibc, this assertion is fired: https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/____longjmp_chk.c;hb=HEAD#l54

Here is a similar commit in glibc tests: https://sourceware.org/git/?p=glibc.git;a=commit;h=46432be2f1d4de962b51ca6b9f80fc37744be9f7

crates/wasmtime/src/runtime/vm/helpers.c

@@ -1,3 +1,8 @@
+// When using _FORTIFY_SOURCE with `longjmp` causes longjmp_chk to be used instead.
+// longjmp_chk ensures that the jump target is on the existing stack. 
+// For our use case of jumping between stacks we need to disable it.
+#undef _FORTIFY_SOURCE
+
 #include <setjmp.h>
 #include <stdint.h>
 #include <stdlib.h>