math: Remove UB and optimize double ilogbf

math: Remove UB and optimize double ilogbf

The subnormal exponent calculation invokes UB by left shifting the
signed expoenent to find the first leading bit.

The patch reimplements ilogb using the math_config.h macros and
uses the new stdbit.h function to simplify the subnormal handling.

On aarch64 it generates better code:

* master:

0000000000000000 <__ieee754_ilogbf>:
   0:   1e260000        fmov    w0, s0
   4:   12007801        and     w1, w0, #0x7fffffff
   8:   72091c1f        tst     w0, #0x7f800000
   c:   54000141        b.ne    34 <__ieee754_ilogbf+0x34>  // b.any
  10:   34000201        cbz     w1, 50 <__ieee754_ilogbf+0x50>
  14:   53185c21        lsl     w1, w1, #8
  18:   12800fa0        mov     w0, #0xffffff82                 // #-126
  1c:   d503201f        nop
  20:   531f7821        lsl     w1, w1, #1
  24:   51000400        sub     w0, w0, #0x1
  28:   7100003f        cmp     w1, #0x0
  2c:   54ffffac        b.gt    20 <__ieee754_ilogbf+0x20>
  30:   d65f03c0        ret
  34:   13177c20        asr     w0, w1, #23
  38:   12b01002        mov     w2, #0x7f7fffff                 // #2139095039
  3c:   5101fc00        sub     w0, w0, #0x7f
  40:   6b02003f        cmp     w1, w2
  44:   12b00001        mov     w1, #0x7fffffff                 // #2147483647
  48:   1a819000        csel    w0, w0, w1, ls  // ls = plast
  4c:   d65f03c0        ret
  50:   320107e0        mov     w0, #0x80000001                 // #-2147483647
  54:   d65f03c0        ret

* patch:

0000000000000000 <__ieee754_ilogbf>:
   0:   1e260001        fmov    w1, s0
   4:   d3577820        ubfx    x0, x1, #23, #8
   8:   350000e0        cbnz    w0, 24 <__ieee754_ilogbf+0x24>
   c:   53175821        lsl     w1, w1, #9
  10:   34000141        cbz     w1, 38 <__ieee754_ilogbf+0x38>
  14:   5ac01021        clz     w1, w1
  18:   12800fc0        mov     w0, #0xffffff81                 // #-127
  1c:   4b010000        sub     w0, w0, w1
  20:   d65f03c0        ret
  24:   7103fc1f        cmp     w0, #0xff
  28:   5101fc00        sub     w0, w0, #0x7f
  2c:   12b00001        mov     w1, #0x7fffffff                 // #2147483647
  30:   1a811000        csel    w0, w0, w1, ne  // ne = any
  34:   d65f03c0        ret
  38:   320107e0        mov     w0, #0x80000001                 // #-2147483647
  3c:   d65f03c0        ret

Other architecture with support for stdc_leading_zeros and/or
__builtin_clzll should have similar improvements.

Checked on aarch64-linux-gnu and x86_64-linux-gnu.