Add MCST Elbrus support

[a1batross]

• Adds MCST Elbrus (e2k) support to zstd. Specifically:
  • adds __e2k__ to the 64-bit platform detection in programs/platform.h
  • enables direct unaligned memory access, since the CPU handles unaligned loads efficiently
  • uses e2k builtins for 32/64-bit rotates and the XXH64 round in xxhash
  • adds __e2k__ to the 64-bit arch check in BIT_getLowerBits: the MCST lcc compiler emulates x86 intrinsics and defines __BMI2__ - without this change the build hits the DEBUG_STATIC_ASSERT in the 32-bit _bzhi_u32 branch;
• Elbrus is a VLIW/EPIC architecture developed by Moscow Center of SPARC Technologies.
• Both make check and make test pass on both x86 and my personal e2k-v3 and e2k-v5 machines.
• To run make test with the MCST lcc compiler, -Wno-error=sign-compare is required due to its stricter sign-compare diagnostics: the warnings only affect asserts, not real code.

[r-a-sattarov]

LGTM 👍