Support _Float64x in libm_alias macros.

Support _Float64x in libm_alias macros.

This patch adds support for libm_alias_ldouble and libm_alias_float128
to create *f64x function aliases when appropriate.

Making such aliases work for functions defined in assembly sources
requires adding some semicolons after weak_alias calls in alias macro
definitions.  For C, semicolons are already present in the macros
called when required, but a GNU C extension allows excess semicolons
at file scope in a source file (and glibc already uses this), so it is
OK to have extra semicolons present in the macro definitions.  For
assembly sources, making multiple alias macro calls from a single
macro expansion means there are no newlines between the calls, so an
explicit separator is needed.  If hppa were to have .S sources in
libm, a more complicated approach would be needed that used
ASM_LINE_SEP when building assembly sources but not for C, but right
now there are no such sources so just using a semicolon (as already
present unconditionally in some such macro expansions) suffices.

Tested for x86_64, including in conjunction with _Float64x support
patches.

	* sysdeps/generic/libm-alias-float128.h: Include <bits/floatn.h>.
	(libm_alias_float128_other_r): If
	[__HAVE_FLOAT64X && !__HAVE_FLOAT64X_LONG_DOUBLE], define f64x
	alias.
	(libm_alias_float128_r): Add semicolon after weak_alias call.
	* sysdeps/generic/libm-alias-ldouble.h
	(libm_alias_ldouble_other_r_f128): New macro.
	(libm_alias_ldouble_other_r_f64x): Likewise.
	(libm_alias_ldouble_other_r): Use libm_alias_ldouble_other_r_f128
	and libm_alias_ldouble_other_r_f64x.
	(libm_alias_ldouble_r): Add semicolon after weak_alias call.
	* sysdeps/ieee754/ldbl-opt/libm-alias-ldouble.h
	(libm_alias_ldouble_other_r_f128): New macro.
	(libm_alias_ldouble_other_r_f64x): Likewise.
	(libm_alias_ldouble_other_r): Use libm_alias_ldouble_other_r_f128
	and libm_alias_ldouble_other_r_f64x.