arch: riscv: Align semihost_exec function at 16-byte boundary

QEMU requires that the semihosting trap instruction sequence, which
consists of three uncompressed instructions, lie in the same page, and
refuses to interpret the trap sequence if these instructions are placed
across two different pages.

This commit adds 16-byte alignment requirement to the `semihost_exec`
function, which occupies 12 bytes, to ensure that the three trap
sequence instructions in this function are never placed across two
different pages.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>