RISC-V: vDSO: Wire up getrandom() vDSO implementation

Hook up the generic vDSO implementation to the generic vDSO getrandom
implementation by providing the required __arch_chacha20_blocks_nostack
and getrandom_syscall implementations. Also wire up the selftests.

The benchmark result:

	vdso: 25000000 times in 2.466341333 seconds
	libc: 25000000 times in 41.447720005 seconds
	syscall: 25000000 times in 41.043926672 seconds

	vdso: 25000000 x 256 times in 162.286219353 seconds
	libc: 25000000 x 256 times in 2953.855018685 seconds
	syscall: 25000000 x 256 times in 2796.268546000 seconds

[ alex: - Fix dynamic relocation
        - Squash Nathan's fix https://lore.kernel.org/all/20250423-riscv-fix-compat_vdso-lld-v2-1-b7bbbc244501@kernel.org/


	- Add comment from Loongarch ]

Signed-off-by: Xi Ruoyao <xry111@xry111.site>
Link: https://lore.kernel.org/r/20250411024600.16045-1-xry111@xry111.site


Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>