Tweaked LFS_ASSERT impl to use __builtin_unreachable

First, realized the the LFS_UNREACHABLE logic was flipped after a
confusing test bug (damn double negatives). But also realized LFS_ASSERT
could be tweaked to "call" __builtin_unreachable() on assert failure to
act as a sort of compiler hint.

Turns out this hint saves a little bit of code, note both builds have
LFS_UNREACHABLE fixed:

                                   code          stack
  without __builtin_unreachable:  28408           1928
  with __builtin_unreachable:     28324 (-0.3%)   1920 (+0.0%)

Since __builtin_unreachable is a compiler extension, its usage respects
LFS_NO_INTRINSICS.

lfs_util.h

@@ -95,6 +95,8 @@ extern "C"
 #ifndef LFS_ASSERT
 #ifndef LFS_NO_ASSERT
 #define LFS_ASSERT(test) assert(test)
+#elif !defined(LFS_NO_INTRINSICS)
+#define LFS_ASSERT(test) ((test) ? (void)0 : __builtin_unreachable())
 #else
 #define LFS_ASSERT(test)
 #endif
@@ -102,9 +104,11 @@ extern "C"
 
 #ifndef LFS_UNREACHABLE
 #ifndef LFS_NO_ASSERT
+#define LFS_UNREACHABLE() LFS_ASSERT(false)
+#elif !defined(LFS_NO_INTRINSICS)
 #define LFS_UNREACHABLE() __builtin_unreachable()
 #else
-#define LFS_UNREACHABLE() LFS_ASSERT(false)
+#define LFS_UNREACHABLE()
 #endif
 #endif