Resolve "comparison of signed and unsigned values" compiler warnings.

Resolve "comparison of signed and unsigned values" compiler warnings.

* cord/cordprnt.c (CORD_vsprintf): Cast "prec" and "width" local
variables to size_t.
* dyn_load.c (GC_register_dynamic_libraries_dl_iterate_phdr): Change
type of "i" local variable to int.
* pthread_support.c (start_mark_threads): Likewise.
* os_dep.c (GC_repeat_read): Change type of "num_read" to size_t.
* os_dep.c (GC_get_maps): Change type of "result" local variable from
int to ssize_t; cast "result" to size_t in comparison.
* pthread_support.c (GC_remove_all_threads_but_me): Cast "result" to
int.
* pthread_support.c (GC_wait_for_gc_completion): Change type of
"old_gc_no" local variable to word.
* pthread_support.c (GC_lock): Change type of "i" local variable to
unsigned.
* tests/staticrootstest.c (main): Cast sizeof() value to int in
comparisons.