Add a config option to guard (future) usage of asm_volatile_goto() that
includes "tied outputs", i.e. "+" constraints that specify both an input
and output parameter.  clang-13 has a bug[1] that causes compilation of
such inline asm to fail, and KVM wants to use a "+m" constraint to
implement a uaccess form of CMPXCHG[2].  E.g. the test code fails with

  <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
  int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
                            ^
  <stdin>:1:29: error: unknown token in expression
  <inline asm>:1:9: note: instantiated into assembly here
          .long () - .
                 ^
  2 errors generated.

on clang-13, but passes on gcc (with appropriate asm goto support).  The
bug is fixed in clang-14, but won't be backported to clang-13 as the
changes are too invasive/risky.

gcc also had a similar bug[3], fixed in gcc-11, where gcc failed to
account for its behavior of assigning two numbers to tied outputs (one
for input, one for output) when evaluating symbolic references.

[1] https://github.com/ClangBuiltLinux/linux/issues/1512
[2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@google.com
[3] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220202004945.2540433-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>