RISC-V: Add implication from the XTheadZvamo extension

RISC-V: Add implication from the XTheadZvamo extension

T-Head/XuanTie's vector extension documentation states that their vector
extensions are based on the standard vector extension draft,
version 0.7.1.

In that draft, it is rare to see dependencies between extensions as
in the ratified version ... except: "Zvamo" -> "Zaamo".

cf. <https://github.com/riscvarchive/riscv-v-spec/releases/tag/0.7.1>

> If vector AMO instructions are supported, then the scalar Zaamo
> instructions (atomic operations from the standard A extension)
> must be present.

Note that using the words like "imply" or "depend" for extensions
weren't a common practice to represent dependencies between extensions
at the time the documentation was created.

The "Zaamo" was not ratified as an extension at the time but this is a
subset of the "A" extension and defines scalar AMO operations (while
"Zvamo" -- NOT in the ratified specification -- defines vector AMO ops).

The important part is that the T-Head/XuanTie's documentation just
states that the "Zvamo" (draft) extension is renamed to "XTheadZvamo".
It means, this implication should have been preserved in some way.

> The extension Zvamo is renamed to XTheadZvamo.

cf. <https://github.com/XUANTIE-RV/thead-extension-spec/blob/2.3.0/xtheadvector.adoc>

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_implicit_subsets): Add implication
	"XTheadZvamo" -> "Zaamo".

gas/ChangeLog:

	* testsuite/gas/riscv/imply.s: Add "XTheadZvamo" implication.
	* testsuite/gas/riscv/imply.d: Ditto.