RISC-V: Add stub support for the 'Svadu' extension

RISC-V: Add stub support for the 'Svadu' extension

This commit implements support for 'Svadu' extension.  Because it does not
add any instructions or CSRs (but adds bits to existing CSRs), this commit
only adds extension name support and implication to the 'Zicsr' extension.

This is based on the "Hardware Updating of PTE A/D Bits (Svadu)"
specification, version 1.0-rc1 (Frozen):
<https://github.com/riscv/riscv-svadu/releases/tag/v1.0-rc1>

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Add implication from
	'Svadu' to 'Zicsr'.  (riscv_supported_std_s_ext) Add 'Svadu'.