arm64: make SPE regs use ALT_NAME macro

When the register is not defined in Armv8.0 i.e. added in a later
extension, like SPE added in v8.2, the alternative name format of:

S<op0>_<op1>_C<crn>_C<crm>_<op2>

should be used; otherwise, calls to {READ,WRITE}_SPECIALREG() will
fail.

Use the MRS_REG_ALT_NAME() macro for SPE changing hex to decimal as
required by the macro.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45171