RISC-V: Change CALL macro to use ra as the temporary address register
    
    e.g.
    
        1:  auipc ra, %pcrel_hi(symbol)
            jalr  ra, %pcrel_lo(1b)(ra)
    
    The use of ra instead of t1 for address construction provides an
    opportunity for a microarchitecture to elide the write of the
    destination address, and instead read the target address as an
    immediate spread across the fused auipc+jalr pair. The link
    register ra in the jalr overwrites the target address temporary.