arm64: make SPE regs use ALT_NAME macro

[zachary.leaf_arm.com]

Summary

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.

Sponsored by: Arm Ltd

[andrew]

sys/arm64/include/armreg.h

1638

As with D45173 keep the existing MRS_REG macro and add a _REG one with MRS_REG_ALT_NAME

[zachary.leaf_arm.com]

sys/arm64/include/armreg.h

1638

Would the non-_REG one ever actually be usable or useful in any way? Trying to understand the logic of keeping the existing ones. MRS_REG produces a 24 bit value e.g. 0x189ae0, the _ALT_NAME one S3_0_C9_C10_7.

[andrew]

sys/arm64/include/armreg.h

1638

They are used in identcpu.c when we need to provide a common value across all CPUs. In this case they are unlikely to be used, but I'd prefer to keep the macro names consistent.

[zachary.leaf_arm.com]

As per review comments