Fix failing gcc.target/avr/torture/builtins_error.c

The compiler defaults to thin LTO objects if built with linker plugin
support, and in that case, the error expected by the testcase appears 
only at link time, if at all. Forcing fat LTO object file creation 
generates the error consistently at compile time, as expected.

gcc/testsuite/ChangeLog:

2016-10-03  Senthil Kumar Selvaraj  <senthil_kumar.selvaraj@atmel.com>

	* gcc.target/avr/torture/builtins-error.c: Add -ffat-lto-objects
	option.

From-SVN: r240711