riscv: align stack in clone [BZ #28702]

The RISC-V ABI [1] mandates that "the stack pointer shall be aligned to
a 128-bit boundary upon procedure entry". This as not the case in clone.

This fixes the misc/tst-misalign-clone-internal and
misc/tst-misalign-clone tests.

Fixes bug 28702.

[1] https://github.com/riscv-non-isa/riscv-elf-psabi-doc