style: use consistently *_loc for locations

Some members are called foo_location, others are foo_loc.  Stick to
the latter.

* src/gram.h, src/location.h, src/location.c, src/output.c,
* src/parse-gram.y, src/reader.h, src/reader.c, src/reduce.c,
* src/scan-gram.l, src/symlist.h, src/symlist.c, src/symtab.h,
* src/symtab.c:
Use _loc consistently, not _location.

src/location.h

@@ -102,7 +102,7 @@ typedef struct
 # define GRAM_LTYPE location
 
 # define EMPTY_LOCATION_INIT {{NULL, 0, 0, 0}, {NULL, 0, 0, 0}}
-extern location const empty_location;
+extern location const empty_loc;
 
 /* Set *LOC and adjust scanner cursor to account for token TOKEN of
    size SIZE.  */