RISC-V: Enable elf attributes when default configure option isn't set.

RISC-V: Enable elf attributes when default configure option isn't set.

Since gcc commit, 3c70b3ca1ef58f302bf8c16d9e7c7bb8626408bf, we now enable
elf attributes for all riscv targets by default in gcc.  Therefore, I
think binutils should have the same behavior, in case users are writing
assembly files.  If --enable-default-riscv-attribute isn't set, then we
enable the elf attributes for all riscv targets by default.

ChangLog:

binutils/

	* testsuite/binutils-all/readelf.s: Add comments for riscv.
	* testsuite/binutils-all/readelf.s-64: Likewise.
	* testsuite/binutils-all/readelf.s-64-unused: Likewise.
	* testsuite/binutils-all/readelf.ss: Likewise.
	* testsuite/binutils-all/readelf.ss-64: Likewise.
	* testsuite/binutils-all/readelf.ss-64-unused: Likewise.

gas/

	* configure.ac: If --enable-default-riscv-attribute isn't set,
	then we enable the elf attributes for all riscv targets by
	default.
	* configure: Regenerated.