Fix missing posix_fadvise64 mips64 static build (BZ #21232)

This patch fixes the missing posix_fadvise64 symbol for static build
required for _FILE_OFFSET_BITS=64 on mips64 build.

Checked on a mips64-linux-gnu build with run-built-tests=no.

	[BZ #21232]
	* sysdeps/unix/sysv/linux/mips/mips64/n64/posix_fadvise64.c: Add
	posix_fadvise64 weak_alias for static build.

(cherry picked from commit ed7d6072)