Remove .weak, .weakext configure tests.

There are configure tests for assembler .weak support, and, as a
fallback, for .weakext support.

.weakext appears to be an ECOFF thing (although a few ELF targets
support it as well).  .weak has been supported by the GNU assembler
for ELF targets since version 2.2, so given the requirement for ELF
the configure tests are obsolete; this patch removes them.

Tested for x86_64 (testsuite, and that installed shared libraries are
unchanged by the patch).

	* configure.ac (libc_cv_asm_weak_directive): Remove configure
	test.
	(libc_cv_asm_weakext_directive): Likewise.
	* configure: Regenerated.
	* config.h.in (HAVE_ASM_WEAK_DIRECTIVE): Remove #undef.
	(HAVE_ASM_WEAKEXT_DIRECTIVE): Likewise.
	* include/libc-symbols.h
	[!HAVE_ASM_WEAK_DIRECTIVE && !HAVE_ASM_WEAKEXT_DIRECTIVE]: Remove
	#error.
	[HAVE_ASM_WEAKEXT_DIRECTIVE]: Remove conditional code.
	[!HAVE_ASM_WEAKEXT_DIRECTIVE]: Make code unconditional.