Build: Enabled unaligned access by default on PowerPC64LE and some RISC-V.

Build: Enabled unaligned access by default on PowerPC64LE and some RISC-V.

PowerPC64LE wasn't tested but it seems like a safe change.
POWER8 supports unaligned access in little endian mode. Testing
on godbolt.org shows that GCC uses unaligned access by default.

The RISC-V macro __riscv_misaligned_fast is very new and not
in any stable compiler release yet.

Documentation in INSTALL was updated to match.

Documentation about an autodetection bug when using ARM64 GCC
with -mstrict-align was added to INSTALL.

CMake files weren't updated yet.