Allow intrinsics to use #[fixed_stack_segment], and use it for numeric intrinsics

This implements the fixed_stack_segment for items with the rust-intrinsic abi, and then uses it to make f32 and f64 use intrinsics where appropriate, but without overflowing stacks and killing canaries (cf. #5686 and #5697). Hopefully.

@pcwalton, the fixed_stack_segment implementation involved mirroring its implementation in `base.rs` in `trans_closure`, but without adding the `set_no_inline` (reasoning: that would defeat the purpose of intrinsics), which is possibly incorrect.

I'm a little hazy about how the underlying structure works, so I've annotated the 4 that caused problems, but there's no guarantee that the other intrinsics won't cause problems.

Anyway, it has good results (the following are just summing the result of each function for 1 up to 100 million):

```
$ ./intrinsics-perf.sh f32
func   new   old   speedup
sin    0.80  2.75  3.44
cos    0.80  2.76  3.45
sqrt   0.56  2.73  4.88
ln     1.01  2.94  2.91
log10  0.97  2.90  2.99
log2   1.01  2.95  2.92
exp    0.90  2.85  3.17
exp2   0.92  2.87  3.12
pow    6.95  8.57  1.23

   geometric mean: 2.97

$ ./intrinsics-perf.sh f64
func   new   old   speedup
sin    12.08  14.06  1.16
cos    12.04  13.67  1.14
sqrt   0.49  2.73  5.57
ln     4.11  5.59  1.36
log10  5.09  6.54  1.28
log2   2.78  5.10  1.83
exp    2.00  3.97  1.99
exp2   1.71  3.71  2.17
pow    5.90  7.51  1.27

   geometric mean: 1.72
```

So about 3x faster on average for f32, and 1.7x for f64.