Fix posix_fadvise64 build on mips64n64

Recent binutils versions (at least 2.27) complains about libc.so
when linking sotruss-lib.so with:

  libc.so:(*IND*+0x0): multiple definition of `posix_fadvise64@GLIBC_2.2'
  libc.so::(.text+0xcf940): first defined here
  libc.so:(*IND*+0x0): multiple definition of `posix_fadvise64'
  collect2: error: ld returned 1 exit status

Dynamic symbols for libc.so shows (readelf --dyn-syms):

   262: 000000000010b950    28 FUNC    GLOBAL DEFAULT   12 posix_fadvise64@GLIBC_2.2
   417: 000000000010b950    28 FUNC    WEAK   DEFAULT   12 posix_fadvise64@@GLIBC_2.2
  1505: 000000000010b950    28 FUNC    GLOBAL DEFAULT   12 posix_fadvise64@@GLIBC_2.3.3

That is, two separate definitions at version GLIBC_2.2. The issue is
sysdeps/unix/sysv/linux/posix_fadvise64.c creates posix_fadvise64 weak_alias,
while sysdeps/unix/sysv/linux/mips/mips64/n64/posix_fadvise64.c then adds
compat_symbol / versioned_symbol calls.

The patch remove the weak_alias definition on mips64 specific version so
direct weak_alias is disabled.

Checked on mips64n64 build with binutils 2.27.51.20161012.

	* sysdeps/unix/sysv/linux/mips/mips64/n64/posix_fadvise64.c:
	Undefine weak_alias.