Fix type of .persistent.bss section

The change implementing the .persistent family of sections broke the
existing support for the .persistent.bss section in the compiler:

int a __attribute__ ((section (".persistent.bss")));

t.s: Assembler messages:
t.s:4: Warning: setting incorrect section type for .persistent.bss

The compiler encodes it as @nobits but the assembler expects @progbits.
The assembler is incorrect and should treat the section like the compiler.

bfd/
	* elf.c (special_sections_p): Add .persistent.bss.
gas/
	* testsuite/gas/elf/section25.d: Run it everywhere.
	* testsuite/gas/elf/section26.d: Likewise.
	* testsuite/gas/elf/section26.s: Add test for .persistent.bss.