Fix PR target/50739

This patch fixes a problem with fmerge-all-constants and the progmem
attribute.

gcc/

	PR target/50739	
	* config/avr/avr.c (avr_asm_select_section): Strip off
	SECTION_DECLARED from flags when calling get_section.

testsuite/

	PR target/50739	
	* gcc.target/avr/pr50739.c: New test

From-SVN: r238041