compiler-rt: Allow build without __c11_atomic_fetch_nand

Don't build atomic fetch nand libcall functions when the required
compiler builtin isn't available. Without this compiler-rt can't be
built with LLVM 13 or earlier.

Not building the libcall functions isn't optimal, but aligns with the
usecase in FreeBSD where compiler-rt from LLVM 14 is built with an LLVM
13 clang and no LLVM 14 clang is built.

Reviewed By:	efriedma

Differential Revision:	https://reviews.llvm.org/D126710

Author: brooksdavis

compiler-rt/lib/builtins/atomic.c

@@ -365,6 +365,9 @@ OPTIMISED_CASES
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW(n, lockfree, type, xor, ^)
 OPTIMISED_CASES
 #undef OPTIMISED_CASE
+// Allow build with clang without __c11_atomic_fetch_nand builtin (pre-14)
+#if __has_builtin(__c11_atomic_fetch_nand)
 #define OPTIMISED_CASE(n, lockfree, type) ATOMIC_RMW_NAND(n, lockfree, type)
 OPTIMISED_CASES
 #undef OPTIMISED_CASE
+#endif