lto: Compile LTO 15 test with -fno-fat-lto-objects

When -ffat-lto-objects is used to compile binutils, LTO 15 test fails on
32-bit targets the same way as -fno-lto, where the builtin function is
used to divide unsigned 64-bit integers.  Compile LTO 15 test with
-fno-fat-lto-objects so that it passes on both 32-bit and 64-bit targets.

	PR ld/33709
	* testsuite/ld-plugin/lto.exp (lto_link_tests): Compile lto-15a.c
	and lto-15b.c with -fno-fat-lto-objects.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>