aix: only create named section for VAR_DECL or FUNCTION_DECL

aix: only create named section for VAR_DECL or FUNCTION_DECL

get_constant_section() can be passed constant-like non-DECLs, such as
CONSTRUCTOR or STRING_CST, which make DECL_SECTION_NAME unhappy
(asserted in symtab_node::get).  This patch ensures that xcoff select
section only invokes resolve_unique_section() for DECLs.

gcc/ChangeLog

2020-07-10  David Edelsohn  <dje.gcc@gmail.com>

	* config/rs6000/rs6000.c (rs6000_xcoff_select_section): Only
	create named section for VAR_DECL or FUNCTION_DECL.