Add .clang-format file; fix ldt test for x64; remove an unused local variable

[IgorKorkin]

• add file .clang-format, as we discussed early fix build warnings and apply code style #291 (comment)
• update .gitignore
• fix test get_ldt_base()for x64
  SLDT instruction fails if If CR4.UMIP = 1 and CPL > 0., see https://www.felixcloutier.com/x86/sldt
  where UMIP - User-Mode Instruction Prevention (if set, #GP on SGDT, SIDT, SLDT, SMSW, and STR instructions when CPL > 0)
• remove an unused local variable from the test lack_user_input()

[ayoubfaouzi]

Hey @IgorKorkin

Thanks for adding that clang-format 👍 , I will later add like a pre-commit-hook or a test in CI to force contributors to format the code.