Fix ecvt_r, fcvt_r namespace (bug 18522).

The functions ecvt, fcvt and gcvt, in some standards, bring in
references to ecvt_r and fcvt_r, which aren't in any of those
standards.  The calls are correctly to __ecvt_r and __fcvt_r, but then
the names ecvt_r and fcvt_r are defined as strong aliases; this patch
changes them to weak aliases.

Tested for x86_64 and x86 (testsuite, and that disassembly of
installed stripped shared libraries is unchanged by the patch).

	[BZ #18522]
	* misc/efgcvt_r.c
	[LONG_DOUBLE_COMPAT (libc, GLIBC_2_0) && !LONG_DOUBLE_CVT]
	(cvt_symbol): Use weak_alias instead of strong_alias.
	[LONG_DOUBLE_COMPAT (libc, GLIBC_2_0)] (cvt_symbol): Likewise.
	* conform/Makefile (test-xfail-XPG4/stdlib.h/linknamespace):
	Remove variable.
	(test-xfail-UNIX98/stdlib.h/linknamespace): Likewise.
	(test-xfail-XOPEN2K/stdlib.h/linknamespace): Likewise.