Fix documentation about math functions on MacOS X.

Fix documentation about math functions on MacOS X.

* doc/posix-functions/exp2.texi: Don't say the function is missing on
MacOS X 10.5.
* doc/posix-functions/fdim.texi: Likewise.
* doc/posix-functions/feclearexcept.texi: Likewise.
* doc/posix-functions/fegetenv.texi: Likewise.
* doc/posix-functions/fegetround.texi: Likewise.
* doc/posix-functions/feholdexcept.texi: Likewise.
* doc/posix-functions/feraiseexcept.texi: Likewise.
* doc/posix-functions/fesetenv.texi: Likewise.
* doc/posix-functions/fesetround.texi: Likewise.
* doc/posix-functions/fetestexcept.texi: Likewise.
* doc/posix-functions/feupdateenv.texi: Likewise.
* doc/posix-functions/fmax.texi: Likewise.
* doc/posix-functions/fmin.texi: Likewise.
* doc/posix-functions/log2.texi: Likewise.
* doc/posix-functions/modff.texi: Likewise.
* doc/posix-functions/nan.texi: Likewise.
* doc/posix-functions/nanf.texi: Likewise.
* doc/posix-functions/nextafterf.texi: Likewise.
* doc/posix-functions/remquo.texi: Likewise.