devel/tup: correctly detect endianness

Fixes build on powerpc64le:

bootstrap LD tup  -fstack-protector-strong  -lm

SQL error: database disk image is malformed
Query was: create table node (id integer primary key not null, dir integer not null, type integer not null, mtime integer not null, srcid integer not null, name varchar(4096), display varchar(4096), flags varchar(256), unique(dir, name))
.tup repository initialized: .tup/db
SQL error: database disk image is malformed
Query was: create table node (id integer primary key not null, dir integer not null, type integer not null, mtime integer not null, srcid integer not null, name varchar(4096), display varchar(4096), flags varchar(256), unique(dir, name))
.tup repository initialized: :memory: