vmlinux.lds.h: remove old check for GCC 4.9

Now that GCC 5.1 is the minimally supported version of GCC, we can
effectively revert commit 85c2ce91 ("sched, vmlinux.lds: Increase
STRUCT_ALIGNMENT to 64 bytes for GCC-4.9")

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>