Add SIMD support to reg_set_value()

Today we have support for mapping a DR_REG_ enum to a dr_mcontext_t field for GPR's for reading and writing in reg_{get,set}_value(), and for SIMD regs for reading in reg_get_value_ex(). This issue covers adding SIMD write support via reg_set_value_ex(). This will be especially useful if the new AVX-512 enums are separated from the existing enums, which is likely for compatibility.