riscv: place traps and fastpath code adjacently

[Hesham Almatary]

Follow Arm's code where it tries to place traps and vector
code adjacently in a 4KiB page to optimise performance
(through spatial cache locality).

This will also allow reducing the number of instructions
in the fastpath code by one by using conditional branches instead
of a check followed by a jump instruction. Though, if the future traps
or fastpath code got bigger than 4 KiB, this may fail linking again.