Fix MIPS n64 posix_fadvise namespace (bug 17796).

sysdeps/unix/sysv/linux/mips/mips64/n64/posix_fadvise.c defines
posix_fadvise64 as a strong alias for posix_fadvise (for
!SHLIB_COMPAT(libc, GLIBC_2_2, GLIBC_2_3_3) - i.e., for static
linking, which is the case when this matters), but it should be a weak
alias.  This patch makes it a weak alias.

Tested for MIPS that this fixes the observed linknamespace test
failures.

	[BZ #17796]
	* sysdeps/unix/sysv/linux/mips/mips64/n64/posix_fadvise.c
	[!SHLIB_COMPAT(libc, GLIBC_2_2, GLIBC_2_3_3)] (posix_fadvise64):
	Define as weak alias not strong alias.