On systems using sysdeps/unix/sysv/linux/wordsize-64, posix_fadvise64
and posix_fallocate64 (non-POSIX) are strong aliases for posix_fadvise
and posix_fallocate (POSIX), meaning references to the latter wrongly
bring in definitions of the former. They should be weak aliases; this
patch makes them so.
Tested for x86_64 (testsuite, and that disassembly of installed shared
libraries is unchanged by the patch).