Remove assembler -mtune=i686 configure test.

Remove assembler -mtune=i686 configure test.

There is a configure test for assembler support for -mtune=i686.  This
option was added in binutils 2.18 so the test is obsolete; this patch
removes it.

Tested for x86 (testsuite, and that installed shared libraries are
unchanged by the patch).

	* sysdeps/i386/configure.ac (libc_cv_as_i686): Remove configure
	test.
	* sysdeps/i386/configure: Regenerated.
	* sysdeps/i386/i686/Makefile [$(config-asflags-i686) = yes]: Make
	code unconditional.