x86 UEFI: Copy code from GNU-EFI

To build x86 test cases with UEFI, we need to borrow some source
code from GNU-EFI, which includes the initialization code and linker
scripts. This commit only copies the source code, without any
modification. These source code files are not used by KVM-Unit-Tests
in this commit.

The following source code is copied from GNU-EFI:
   1. x86/efi/elf_x86_64_efi.lds
   2. x86/efi/reloc_x86_64.c
   3. x86/efi/crt0-efi-x86_64.S

We put these EFI-related files under a new dir `x86/efi` because:
   1. EFI-related code is easy to find
   2. EFI-related code is separated from the original code in `x86/`
   3. EFI-related code can still reuse the Makefile and test case code
      in its parent dir `x86/`

GNU-EFI repo and version:
   GIT URL: https://git.code.sf.net/p/gnu-efi/code
   Commit ID: 4fe83e102674
   Website: https://sourceforge.net/p/gnu-efi/code/ci/4fe83e/tree/



Co-developed-by: Varad Gautam <varad.gautam@suse.com>
Signed-off-by: Varad Gautam <varad.gautam@suse.com>
Signed-off-by: Zixuan Wang <zixuanwang@google.com>
Message-Id: <20211004204931.1537823-5-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>