- Add implementation for x86_64 and linux
- Add test

The output is like
`
==XXYYZZ==Register values:
rax = 0x...  rbx = 0x...  rcx = 0x...  rdx = 0x...
rdi = 0x...  rsi = 0x...  rbp = 0x...  rsp = 0x...
 r8 = 0x...   r9 = 0x...  r10 = 0x...  r11 = 0x...
r12 = 0x...  r13 = 0x...  r14 = 0x...  r15 = 0x...
`

Fox example gcc warns about `RegNumToRegName` is unused
and about `ucontext` is unused

- Restore dump_registers.cpp test in Posix/ dir
- Add linux-specifix tests inside Linux/ directory
Mac uses single dump_registers.cpp test inside Darwin/ directory
though checks in this test are partially 'restricted'
For linux we provide one test per each supported architecture with
checks for each part of `DumpAllRegisters` output