Use a constant pointer to fetch constants

[davidharrishmc]

Using li is a bit cumbersome to fetch constants, especially for RV64. Floating-point constants are worse. Simplify this by keeping the constants in program memory and reading them from a constant pointer.

See the end of
https://docs.google.com/document/d/1diwgsQUAIEIrcQ8W-Sqkbh0SXIaVyWhoZUS2kEj24jg/edit?tab=t.0
for an example.

testgen.py will need to provide a constant pointer register that rotates like the signature pointer. li will be replaced with loadIntReg. loadIntReg and loadFloatReg should both reference the signature. They should produce comments so it is easy to know what value is being loaded.