Don't use volatile in exp2f.

sysdeps/ieee754/flt-32/e_exp2f.c declares two variable as "static
const volatile float".  Maybe this use of "volatile" was originally
intended to inhibit optimization of underflowing / overflowing
operations such as TWOM100 * TWOM100; in any case, it's not currently
needed, as given -frounding-math constant folding of such expressions
is properly disabled when it would be unsafe.  This patch removes the
unnecessary use of "volatile".

Tested for x86_64.

	* sysdeps/ieee754/flt-32/e_exp2f.c (TWOM100): Remove volatile.
	(TWO127): Likewise.