math: Fix incorrect results of exp10m1f with some GCC versions

math: Fix incorrect results of exp10m1f with some GCC versions

On GCC 11 (x86-64), the previous code produced test failures like
this one:

Failure: Test: exp10m1_towardzero (-0x1.1p+4)
Result:
 is:         -1.00000000e+00  -0x1.000000p+0
 should be:  -9.99999940e-01  -0x1.fffffep-1
 difference:  5.96046447e-08   0x1.000000p-24
 ulp       :  1.0000
 max.ulp   :  0.0000

Apply a similar fix to exp2m1f.

Co-authored-by: Paul Zimmermann <Paul.Zimmermann@inria.fr>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>