libc: posix_fadvise: Fix build breakage for !LFS

commit 00571b43df2e "libc: posix_fadvise: restore implementation for xtensa"
enabled posix_fadvise() for all arches (it was just not generated
before).

However this also unearthed an issue introduced by ee84b8b400
"linux: posix_fadvise: use new SYSCALL_ALIGN_64BIT" which is to
referencing LFS'ish code (off64_t) w/o proper checks which causes build
to break for !LFS.

Fix this by calling posix_fadvise64() only for LFS case and open-code
it's equivalent for !LFS.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Cc: Mike Frysinger <vapier@gentoo.org>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Khem Raj <raj.khem@gmail.com>
Cc: markos Chandras <markos.chandras@gmail.com>
Cc: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>