Adjust the V calling convention to aid lazy binding

The whole point of this VARIANT_CC bit is that we want to be able to use
V registers in _dl_resolve without saving them.  That requires that the
interface between the caller and the callee does not rely on the values
in the V registers.  That's a bit different than how we've histrorically
defined these temporary register clobbers, which is about the the
caller-side visibility into register changes.

Signed-off-by: Palmer Dabbelt <[email protected]>

---

I'm not entirely sure what the right way to describe this one is.  What
I've got here feels a bit clunky and doesn't call out the lazy binding
issue, but at least it's something.  It also only applies to the V
register calling convention, as it's not clear if the goal here is to
retroactively make this apply to X and F.

riscv-cc.adoc

@@ -59,7 +59,7 @@ duration in accordance with C11 section 7.6 "Floating-point environment
 .Vector register convention
 [%autowidth]
 |===
-| Name    | ABI Mnemonic | Meaning                      | Preserved across calls?
+| Name    | ABI Mnemonic | Meaning                      | Preserved into calls?
 
 | v0-v31  |              | Temporary registers          | No
 | vl      |              | Vector length                | No

riscv-elf.adoc

@@ -134,12 +134,9 @@ Any functions that use registers in a way that is incompatible with
 the calling convention of the ABI in use must be annotated with
 `STO_RISCV_VARIANT_CC`, as defined in <<Symbol Table>>.
 
-NOTE: Vector registers have a variable size depending on the hardware
-implementation and can be quite large. Saving/restoring all these vector
-arguments in a run-time linker's lazy resolver would use a large amount of
-stack space and hurt performance. This attribute allows vector registers to
-not be part of the standard calling convention so run-time linkers are not
-required to save/restore them and can instead eagerly bind such functions.
+NOTE: The standard caling conventions all clobber the V registers on calls, so
+this allows for the direct use (ie, without an expensive save/restore) of those
+registers while resolving lazily bound functions.
 
 == C++ Name Mangling