x86/sev: Work around broken noinstr on GCC

Forcibly disable KCSAN for the sev-nmi.c source file, which only
contains functions annotated as 'noinstr' but is emitted with calls to
KCSAN instrumentation nonetheless. E.g.,

  vmlinux.o: error: objtool: __sev_es_nmi_complete+0x58: call to __kcsan_check_access() leaves .noinstr.text section
  make[2]: *** [/usr/local/google/home/ardb/linux/scripts/Makefile.vmlinux_o:72: vmlinux.o] Error 1

Fixes: a3cbbb47 ("x86/boot: Move SEV startup code into startup/")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/20250714073402.4107091-2-ardb+git@google.com