Cannot build the ACRN Preempt-RT kernel on recent distros (e.g. Ubuntu 20.04)

Newer versions of gcc (such as shipped in Ubuntu 20.04) cannot build the ACRN PREEMPT-RT kernel following these instructions:

$ git checkout https://github.com/projectacrn/acrn-kernel
$ cd acrn-kernel
$ git checkout origin/4.19/preempt-rt
$ make mrproper
$ cp x86-64_defconfig .config
$ make olddefconfig
$ make targz-pkg

Error thrown (not the full log):

LD [M]  arch/x86/crypto/morus1280-sse2.o
  AS [M]  arch/x86/crypto/cast5-avx-x86_64-asm_64.o
  CC [M]  arch/x86/crypto/cast5_avx_glue.o
  LD [M]  arch/x86/crypto/cast5-avx-x86_64.o
  AS [M]  arch/x86/crypto/cast6-avx-x86_64-asm_64.o
  CC [M]  arch/x86/crypto/cast6_avx_glue.o
  LD [M]  arch/x86/crypto/cast6-avx-x86_64.o
  AS [M]  arch/x86/crypto/morus1280-avx2-asm.o
  CC [M]  arch/x86/crypto/morus1280-avx2-glue.o
  LD [M]  arch/x86/crypto/morus1280-avx2.o
  CC      arch/x86/entry/vdso/vma.o
  CC      arch/x86/entry/vdso/vdso32-setup.o
  LDS     arch/x86/entry/vdso/vdso.lds
  AS      arch/x86/entry/vdso/vdso-note.o
  CC      arch/x86/entry/vdso/vclock_gettime.o
In file included from ./arch/x86/include/asm/vgtod.h:5,
                 from arch/x86/entry/vdso/vclock_gettime.c:15:
./include/linux/compiler.h: In function ‘__read_once_size’:
./include/linux/compiler.h:192:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
  192 | {
      | ^
./include/linux/compiler.h: In function ‘__read_once_size_nocheck’:
./include/linux/compiler.h:210:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
  210 | {
      | ^
./include/linux/compiler.h: In function ‘__write_once_size’:
./include/linux/compiler.h:215:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
  215 | {
      | ^
In file included from ./include/linux/compiler.h:250,
                 from ./arch/x86/include/asm/vgtod.h:5,
                 from arch/x86/entry/vdso/vclock_gettime.c:15:
./arch/x86/include/asm/barrier.h: In function ‘array_index_mask_nospec’:
./arch/x86/include/asm/barrier.h:38:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
   38 | {
      | ^
In file included from ./include/linux/compiler.h:251,
                 from ./arch/x86/include/asm/vgtod.h:5,
                 from arch/x86/entry/vdso/vclock_gettime.c:15:
./include/linux/kasan-checks.h: In function ‘kasan_check_read’:
./include/linux/kasan-checks.h:10:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
   10 | { }
      | ^
./include/linux/kasan-checks.h: In function ‘kasan_check_write’:
./include/linux/kasan-checks.h:12:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
   12 | { }
      | ^
In file included from ./arch/x86/include/asm/vgtod.h:5,
                 from arch/x86/entry/vdso/vclock_gettime.c:15:
./include/linux/compiler.h: In function ‘read_word_at_a_time’:
./include/linux/compiler.h:273:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
  273 | {
      | ^
./include/linux/compiler.h: In function ‘offset_to_ptr’:
./include/linux/compiler.h:303:1: error: ‘-mindirect-branch’ and ‘-fcf-protection’ are not compatible
  303 | {
      | ^

projectacrn / acrn-hypervisor

Cannot build the ACRN Preempt-RT kernel on recent distros (e.g. Ubuntu 20.04) #6223