Fix MIPS n32 lseek, lseek64 (bug 21019).

The lseek consolidation broke lseek64 for MIPS n32, so resulting in
io/test-lfs failing with an incorrect return from ftello64.  This
configuration uses the lseek syscall with a 64-bit return value; as
the C syscall macros return long, they cannot be used in this case and
so an assembly implementation is needed; accordingly, this patch adds
lseek64 back to syscalls.list for this configuration.

lseek was also broken, truncating the result without checking for
overflow.  lseek however was already broken before the consolidation;
it aliased lseek64 so would return an out-of-range value, resulting in
architecturally undefined behavior in the caller if it tried to use a
non-sign-extended value with a 32-bit instruction.  This patch adds a
custom lseek implementation in C for n32, which calls __lseek64 to get
the 64-bit value then checks for overflow.

Because the prior lseek breakage did not show in test results, and the
lseek64 breakage showed only indirectly through tests of ftello64,
test coverage was clearly inadequate.  This patch extends
io/test-lfs.c to test the lseek64 return value (at a point where it
has already seeked over 2GB into a file), and then to test the lseek
return value (with the latter's expectations depending on whether
off_t is smaller than off64_t).

Tested for mips64 n32.  Also tested test-lfs for x86_64 and x86, where
as expected it passes.

	[BZ #21019]
	* sysdeps/unix/sysv/linux/mips/mips64/n32/syscalls.list (lseek64):
	New syscall entry.
	* sysdeps/unix/sysv/linux/mips/mips64/n32/lseek.c: New file.
	* io/test-lfs.c (do_test): Test offset returned from lseek64 and
	lseek.

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