riscv vmm: consider hart_mask_base argument in the SBI IPI handler.

From the spec:

Any SBI function, requiring a hart mask, must take the following
two arguments:

• unsigned long hart_mask is a scalar bit-vector containing hartids
• unsigned long hart_mask_base is the starting hartid from which the bit-vector must be computed.

hart_mask_base can be set to -1 to indicate that hart_mask shall be
ignored and all available harts must be considered.

Differential Revision: https://reviews.freebsd.org/D48717