The "cc" clobber in GNU extended asm is dropped with a warning #256

Severity: warning noise on x86/x86-64; a possible wrong-code risk on targets
where GCC does not implicitly treat asm as clobbering the condition codes
(not verified here).
Component: asm_clobbers_spec in src/extasm.c (ec_cc_clobber_ignored).

Reproducer

//options_all:--gnu_version=150000 --diag_error=cc_clobber_ignored
int add_flags(int a, int b) {
  __asm__ __volatile__("addl %1, %0" : "+r"(a) : "r"(b) : "cc");
  return a;
}
void inc(int volatile &x) {
  __asm__ __volatile__("lock; incl %0" : "+m"(x) : : "cc", "memory");
}

Actual (--gnu_version=150301; the same in C mode):

"cc.cpp", line 3: warning: "cc" clobber ignored
    __asm__ __volatile__("addl %1, %0" : "+r"(a) : "r"(b) : "cc");
                                                            ^
"cc.cpp", line 7: warning: "cc" clobber ignored
    __asm__ __volatile__("lock; incl %0" : "+m"(x) : : "cc", "memory");
                                                       ^

The clobber is also missing from the generated C:

__asm__ volatile("addl %1, %0" : "+r" (__2_19_a) : "r" (__2_26_b));

Expected: no diagnostic, and "cc" kept in the generated asm. GCC
(-Wall -Wextra) and Clang accept it silently. GCC documents "cc" as valid
on every target: on targets that model the condition codes as a register it
names that register, and elsewhere it has no effect.

Analysis

} else if (strcmp(name, "cc") == 0) {
  pos_warning(ec_cc_clobber_ignored, &error_position);
  goto skip_item;

The clobber isn't recorded at all, so the C-generating back end can't emit it.
On x86/x86-64 GCC treats every asm statement as clobbering the flags, so the
generated code is unaffected and this is just noise. On a target where GCC
relies on an explicit "cc" to know that the flags change, dropping it could
let GCC keep a comparison result live across the asm. I did not test this,
since this build is x86-64 only. Boost.Atomic uses "cc" heavily in its
ARM/AArch64/PowerPC/SPARC asm backends.

Suggested fix: record "cc" (for example as an anr_cc pseudo-register like
anr_memory), emit it in the C-generating back end, and drop the warning.
At minimum, don't warn in GNU modes.

Impact on Boost

78 warnings in 19 translation units, all from
boost/atomic/detail/extra_ops_gcc_x86.hpp (lines 1079, 1089, 1155,
BOOST_ATOMIC_DETAIL_ASM_CLOBBER_CC_COMMA). They show up in Boost.Atomic and
in libraries that include its extra ops (Thread, Log, Fiber, ...).