Disable __libcpp_verbose_abort directly in code

In #20707 we decided to disable `__libcpp_verbose_abort` not to incur
the code size increase that brings (it brings in `vfprintf` and its
family). We disabled it in adding
`#define _LIBCPP_AVAILABILITY_HAS_NO_VERBOSE_ABORT` in `__config_site`.

That `_NO_` macros are gone in LLVM 18.1.2, so I changed it to
`#define _LIBCPP_AVAILABILITY_HAS_VERBOSE_ABORT 0`
2bd7c67
But apparently, when including `__availability`, that setting is
overridden by this line:
https://github.com/llvm/llvm-project/blob/38f5596feda3276a8aa64fc14e074334017088ca/libcxx/include/__availability#L138
because we don't define our own vendor availability annotations
https://github.com/emscripten-core/emscripten/blob/193b2bff980529dbf5d067f01761ed8a6e41189f/system/lib/libcxx/include/__config_site#L4

I asked about this in
llvm/llvm-project#87012 and
llvm/llvm-project#71002 (comment)
(and more comments below)
but didn't get an actionable answer yet.

This disables `__libcpp_verbose_abort` in the code directly for now,
hopefully temporarily.

Author: aheejin

system/lib/libcxx/include/__config_site

@@ -6,7 +6,5 @@
 #define _LIBCPP_ABI_NAMESPACE __2
 // Emscripten doesn't use PSTL at the moment.
 #define _LIBCPP_PSTL_CPU_BACKEND_SERIAL
-// __libcpp_verbose_abort creases code size too much
-#define _LIBCPP_AVAILABILITY_HAS_VERBOSE_ABORT 0
 #define _LIBCPP_HARDENING_MODE _LIBCPP_HARDENING_MODE_NONE
 #define _LIBCPP_HAS_NO_TIME_ZONE_DATABASE

system/lib/libcxx/include/__verbose_abort

@@ -38,7 +38,8 @@ _LIBCPP_NORETURN _LIBCPP_AVAILABILITY_VERBOSE_ABORT _LIBCPP_OVERRIDABLE_FUNC_VIS
 // make sure that the program terminates but without taking any complex dependencies in this header.
 #if !defined(_LIBCPP_VERBOSE_ABORT)
 
-#  if !_LIBCPP_AVAILABILITY_HAS_VERBOSE_ABORT
+// XXX EMSCRIPTEN __libcpp_verbose_abort creases code size too much
+#  if !_LIBCPP_AVAILABILITY_HAS_VERBOSE_ABORT || defined (__EMSCRIPTEN__)
 // The decltype is there to suppress -Wunused warnings in this configuration.
 void __use(const char*, ...);
 #    define _LIBCPP_VERBOSE_ABORT(...) (decltype(::std::__use(__VA_ARGS__))(), __builtin_abort())