riscv: hwprobe: Add thead vendor extension probing

Add a new hwprobe key "RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0" which
allows userspace to probe for the new RISCV_ISA_VENDOR_EXT_XTHEADVECTOR
vendor e

riscv: hwprobe: Add thead vendor extension probing

Add a new hwprobe key "RISCV_HWPROBE_KEY_VENDOR_EXT_THEAD_0" which
allows userspace to probe for the new RISCV_ISA_VENDOR_EXT_XTHEADVECTOR
vendor extension.

This new key will allow userspace code to probe for which thead vendor
extensions are supported. This API is modeled to be consistent with
RISCV_HWPROBE_KEY_IMA_EXT_0. The bitmask returned will have each bit
corresponding to a supported thead vendor extension of the cpumask set.
Just like RISCV_HWPROBE_KEY_IMA_EXT_0, this allows a userspace program
to determine all of the supported thead vendor extensions in one call.

Signed-off-by: Charlie Jenkins <[email protected]>
Reviewed-by: Evan Green <[email protected]>
Tested-by: Yangyu Chen <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>

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