Add a new precision adjustment for i586 rint and exp2f

[tgross35]

rint had a couple recent failures from the random tests:

---- mp_random_rint stdout ----
Random Mpfr rint arg 1/1: 10000 iterations (10000 total) using `LIBM_SEED=Fl1f69DaJnwkHN2FeuCXaBFRvJYsPvEY`

thread 'mp_random_rint' panicked at crates/libm-test/tests/multiprecision.rs:41:49:
called `Result::unwrap()` on an `Err` value:
    input:    (-849751480.5001163,) (0xc1c95316dc4003d0,)
    expected: -849751481.0           0xc1c95316dc800000
    actual:   -849751480.0           0xc1c95316dc000000

Caused by:
    ulp 8388608 > 100000

And:

---- mp_random_rint stdout ----
Random Mpfr rint arg 1/1: 10000 iterations (10000 total) using `LIBM_SEED=XN7VCGhX3Wu6Mzn8COvJPITyZlGP7gN7`

thread 'mp_random_rint' panicked at crates/libm-test/tests/multiprecision.rs:41:49:
called `Result::unwrap()` on an `Err` value:
    input:    (-12493089.499809155,) (0xc167d4242ffe6fc5,)
    expected: -12493089.0            0xc167d42420000000
    actual:   -12493090.0            0xc167d42440000000

Caused by:
    ulp 536870912 > 100000

It seems we just implement an incorrect rounding mode. Replace the existing rint override with an xfail if the difference is 0.0 <= ε <= 1.0.

---

There was a recent failure from the random tests:

 ---- mp_random_exp2f stdout ----
Random Mpfr exp2f arg 1/1: 10000 iterations (10000 total) using `LIBM_SEED=fqgMuzs6eqH1VZSEmQpLnThnaIyRUOWe`

thread 'mp_random_exp2f' panicked at crates/libm-test/tests/multiprecision.rs:41:49:
called `Result::unwrap()` on an `Err` value:
    input:    (127.97238,) (0x42fff1dc,)
    expected: 3.3383009e38           0x7f7b2556
    actual:   inf                    0x7f800000

Caused by:
    mismatched infinities

Add an xfail for mismatched infinities on i586.