fix broken lseek on mipsn32 with offsets larger than LONG_MAX

mips n32 has 32-bit long, and generally uses long syscall arguments
and return values, but provides only SYS_lseek, not SYS_llseek. we
have some framework (syscall_arg_t, added for x32) to make syscall
arguments 64-bit in such a setting, but it's not clear whether this
could match the sign-extension semantics needed for 32-bit args to all
the other syscalls, and we don't have any existing mechanism to allow
the return value of syscalls to be something other than long.

instead, just provide a custom mipsn32 version of the lseek function
doing its own syscall asm with 64-bit arguments. as a result of commit
03919b26ed, stdio will also get the new
code, fixing fseeko/ftello too.

src/unistd/mipsn32/lseek.c

@@ -0,0 +1,20 @@
+#include <unistd.h>
+#include "syscall.h"
+
+off_t __lseek(int fd, off_t offset, int whence)
+{
+	register long long r4 __asm__("$4") = fd;
+	register long long r5 __asm__("$5") = offset;
+	register long long r6 __asm__("$6") = whence;
+	register long long r7 __asm__("$7");
+	register long long r2 __asm__("$2") = SYS_lseek;
+	__asm__ __volatile__ (
+		"syscall"
+		: "+&r"(r2), "=r"(r7)
+		: "r"(r4), "r"(r5), "r"(r6)
+		: SYSCALL_CLOBBERLIST);
+	return r7 ? __syscall_ret(-r2) : r2;
+}
+
+weak_alias(__lseek, lseek);
+weak_alias(__lseek, lseek64);