Fix regoff_t for LP32 and _FILE_OFFSET_BITS=64.

bionic is built without _FILE_OFFSET_BITS=64, so internally regoff_t
was 32-bit on LP32, but code compiled with _FILE_OFFSET_BITS would
expect rm_so and rm_eo in struct regmatch_t to be 64-bit, leading to
confusion.

Bug: http://b/23566443
Change-Id: Iae92fa545104068e4f64ce1977f5ec616859638c

libc/include/regex.h

@@ -42,8 +42,9 @@
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
-/* types */
-typedef off_t regoff_t;
+/* POSIX says regoff_t is at least as large as the larger of ptrdiff_t and
+ * ssize_t. BSD uses off_t, but that interacts badly with _FILE_OFFSET_BITS. */
+typedef ssize_t regoff_t;
 
 typedef struct {
 	int re_magic;

tests/regex_test.cpp

@@ -36,3 +36,13 @@ TEST(regex, smoke) {
 
   regfree(&re);
 }
+
+TEST(regex, match_offsets) {
+  regex_t re;
+  regmatch_t matches[1];
+  ASSERT_EQ(0, regcomp(&re, "b", 0));
+  ASSERT_EQ(0, regexec(&re, "abc", 1, matches, 0));
+  ASSERT_EQ(1, matches[0].rm_so);
+  ASSERT_EQ(2, matches[0].rm_eo);
+  regfree(&re);
+}