objtool: Ignore __pi___cfi_ prefixed symbols

When building with CONFIG_CFI_CLANG=y after the recent series to
separate the x86 startup code, there are objtool warnings along the
lines of:

  vmlinux.o: warning: objtool: __pi___cfi_startup_64_load_idt() falls through to next function __pi_startup_64_load_idt()
  vmlinux.o: warning: objtool: __pi___cfi_startup_64_setup_gdt_idt() falls through to next function __pi_startup_64_setup_gdt_idt()
  vmlinux.o: warning: objtool: __pi___cfi___startup_64() falls through to next function __pi___startup_64()

As the comment in validate_branch() states, this is expected, so ignore
these symbols in the same way that __cfi_ and __pfx_ symbols are already
ignored for the rest of the kernel.

Fixes: 7b38dec3 ("x86/boot: Create a confined code area for startup code")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Ard Biesheuvel <ardb@kernel.org>