add support for ugly *64 functions with _LARGEFILE64_SOURCE

musl does not support legacy 32-bit-off_t whatsoever. off_t is always
64 bit, and correct programs that use off_t and the standard functions
will just work out of the box. (on glibc, they would require
-D_FILE_OFFSET_BITS=64 to work.) however, some programs instead define
_LARGEFILE64_SOURCE and use alternate versions of all the standard
types and functions with "64" appended to their names.

we do not want code to actually get linked against these functions
(it's ugly and inconsistent), so macros are used instead of prototypes
with weak aliases in the library itself. eventually the weak aliases
may be added at the library level for the sake of using code that was
originally built against glibc, but the macros will still be the
desired solution in the headers.

include/unistd.h

@@ -168,6 +168,16 @@ void endusershell(void);
 char *getusershell(void);
 #endif
 
+#ifdef _LARGEFILE64_SOURCE
+#define lseek64 lseek
+#define pread64 pread
+#define pwrite64 pwrite
+#define truncate64 truncate
+#define ftruncate64 ftruncate
+#define lockf64 lockf
+#define off64_t off_t
+#endif
+
 #define _XOPEN_VERSION          700
 #define _XOPEN_UNIX             1
 #define _XOPEN_ENH_I18N         1