x86 UEFI: Boot from UEFI

This commit provides initial support for x86 test cases to boot from
UEFI:

   1. UEFI compiler flags are added to Makefile
   2. A new TARGET_EFI macro is added to turn on/off UEFI startup code
   3. Previous Multiboot setup code is refactored and updated for
      supporting UEFI, including the following changes:
      1. x86/efi/crt0-efi-x86_64.S: provides entry point and jumps to
         setup code in lib/efi.c.
      2. lib/efi.c: performs UEFI setup, calls arch-related setup
         functions, then jumps to test case main() function
      3. lib/x86/setup.c: provides arch-related setup under UEFI

To build test cases for UEFI, please first install the GNU-EFI library.
Check x86/efi/README.md for more details.

This commit is tested by a simple test calling report() and
report_summayr(). This commit does not include such a test to avoid
unnecessary files added into git history. To build and run this test in
UEFI (assuming file name is x86/dummy.c):

   ./configure --target-efi
   make x86/dummy.efi
   ./x86/efi/run ./x86/dummy.efi

To use the default Multiboot instead of UEFI:

   ./configure
   make x86/dummy.flat
   ./x86/run ./x86/dummy.flat

Some x86 test cases require additional fixes to work in UEFI, e.g.,
converting to position independent code (PIC), setting up page tables,
etc. This commit does not provide these fixes, so compiling and running
UEFI test cases other than x86/dummy.c may trigger compiler errors or
QEMU crashes. These test cases will be fixed by the follow-up commits in
this series.

The following code is ported from github.com/rhdrjones/kvm-unit-tests
   - ./configure: 'target-efi'-related code

See original code:
   - Repo: https://github.com/rhdrjones/kvm-unit-tests


   - Branch: target-efi

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: <20211004204931.1537823-6-zxwang42@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>