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).