Workaround a C++ bug in GCC 4.2 in gold test

Workaround a C++ bug in GCC 4.2 in gold test

G++ in GCC 4.2 silently ignores

---
__attribute__ ((section(".rodata.v1_a2")))
const short rodata_item1 = 101;
---

which leads to plugin_layout_with_alignment test failure with

Expected 12 sections, found 8 sections

since 4 .rodata sections are missing.  As a workaround, this patch
changes plugin_layout_with_alignment test from C++ to C.

	* testsuite/plugin_layout_with_alignment.cc: Renamed to ..
	* testsuite/plugin_layout_with_alignment.c: This.
	* testsuite/Makefile.am (plugin_layout_with_alignment.o): Updated.
	(plugin_layout_with_alignment): Likewise.
	* testsuite/Makefile.in: Regenerated.