alpha/boot/tools/objstrip: fix the check for ELF header

Just memcmp() with ELFMAG - that's the normal way to do it in userland
code, which that thing is.  Besides, that has the benefit of actually
building - str_has_prefix() is *NOT* present in <string.h>.

Fixes: 5f14596e "alpha: Replace strncmp with str_has_prefix"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Matt Turner <mattst88@gmail.com>