Allow configuring Crucible's lax-pointer-ordering option from SAW

[RyanGlScott]

There are some scenarios where LLVM is prone to generate code that compares pointers from different allocation blocks. For example, LLVM's strength reduction passes can generate comparisons between pointers with -O2 or higher, as seen in GaloisInc/crucible#149. Here is another example, distilled from BoringSSL:

#include <stdint.h>
#include <stdlib.h>

uint64_t constant_time_select_w(uint64_t mask, uint64_t a, uint64_t b) {
  return (mask & a) | (~mask & b);
}

void bn_select_words(uint64_t *r, uint64_t mask, const uint64_t *a, const uint64_t *b, size_t num) {
  for (size_t i = 0; i < num; i++) {
    r[i] = constant_time_select_w(mask, a[i], b[i]);
  }
}

These examples work when Crucible's lax-pointer-ordering option is enabled. This option is off by default, however, and SAW does not offer a way to change the default setting. We should offer functionality to do so, much like we offer functionality to configure lax-arithmetic.