Support auto-detection of MAX_REAL_{FLOAT,DOUBLE}_REG up to 6 (#17953)
This MR updates auto-detection logic of MAX_REAL_FLOAT_REG and MAX_REAL_DOUBLE_REG in MachRegs.h to support up to 6 registers each.
Notably, MAX_REAL_DOUBLE_REG will be increased from 2 to 4 on AArch64 (fixing #17953 (closed)).
A test is added to check that four FP registers are used when using foreign import prim on AArch64, by implementing the foreign functions in assembly.