riscv: Fix alignment-ignorant memcpy implementation

riscv: Fix alignment-ignorant memcpy implementation

The memcpy optimization (commit 587a1290a1af7bee6db) has a series
of mistakes:

  - The implementation is wrong: the chunk size calculation is wrong
    leading to invalid memory access.

  - It adds ifunc supports as default, so --disable-multi-arch does
    not work as expected for riscv.

  - It mixes Linux files (memcpy ifunc selection which requires the
    vDSO/syscall mechanism)  with generic support (the memcpy
    optimization itself).

  - There is no __libc_ifunc_impl_list, which makes testing only
    check the selected implementation instead of all supported
    by the system.

This patch also simplifies the required bits to enable ifunc: there
is no need to memcopy.h; nor to add Linux-specific files.

The __memcpy_noalignment tail handling now uses a branchless strategy
similar to aarch64 (overlap 32-bits copies for sizes 4..7 and byte
copies for size 1..3).

Checked on riscv64 and riscv32 by explicitly enabling the function
on __libc_ifunc_impl_list on qemu-system.

Changes from v1:
* Implement the memcpy in assembly to correctly handle RISCV
  strict-alignment.
Reviewed-by: Evan Green <evan@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>