i64: fix missing exp2f, log2f and powf symbols in libm.a [BZ #23822]

When new symbol versions were introduced without SVID compatible
error handling the exp2f, log2f and powf symbols were accidentally
removed from the ia64 lim.a.  The regression was introduced by
the commits

f5f0f526
New expf and exp2f version without SVID compat wrapper

72d3d281
New symbol version for logf, log2f and powf without SVID compat

With WEAK_LIBM_ENTRY(foo), there is a hidden __foo and weak foo
symbol definition in both SHARED and !SHARED build.

	[BZ #23822]
	* sysdeps/ia64/fpu/e_exp2f.S (exp2f): Use WEAK_LIBM_ENTRY.
	* sysdeps/ia64/fpu/e_log2f.S (log2f): Likewise.
	* sysdeps/ia64/fpu/e_exp2f.S (powf): Likewise.