RISC-V: Allow nested implications for extensions

RISC-V: Allow nested implications for extensions

Certain extensions require two levels of implications.  For example,
zvkng implies zvkn and zvkn implies zvkned.  Enabling zvkng should also
enable zvkned.

This patch fixes this behavior.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_add_implicit_subsets): Allow nested
	implications for extensions.

Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>