Fix output_constructor_bitfield handling of wide bitfields (PR89037)

Fix output_constructor_bitfield handling of wide bitfields (PR89037)

The testcase was failing because we were trying to access
TREE_INT_CST_ELT (x, 1) of a 128-bit integer that was small enough
to need only a single element.

2019-01-25  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR middle-end/89037
	* varasm.c (output_constructor_bitfield): Use wi::extract_uhwi
	instead of accessing TREE_INT_CST_ELT directly.

gcc/testsuite/
	PR middle-end/89037
	* gcc.dg/pr89037.c: New test.

From-SVN: r268272