build: use _LIBCPP_ENABLE_DEBUG_MODE over ENABLE_ASSERTIONS

`_LIBCPP_ENABLE_ASSERTIONS` is deprecated, and will be removed. [See (from libc++ __config in main)](https://github.com/llvm/llvm-project/blob/b57df9fe9a1a230f277d671bfa0884bbda9fc1c5/libcxx/include/__config#L205-L209):

> TODO(hardening): remove this in LLVM 19.
> This is for backward compatibility -- make enabling `_LIBCPP_ENABLE_ASSERTIONS` (which predates hardening modes)
> equivalent to setting the safe mode.
> ifdef _LIBCPP_ENABLE_ASSERTIONS
> warning "_LIBCPP_ENABLE_ASSERTIONS is deprecated, please use _LIBCPP_ENABLE_SAFE_MODE instead."

From LLVM 17, `_LIBCPP_ENABLE_DEBUG_MODE` can be used instead.

See https://libcxx.llvm.org/Hardening.html.

Related to #28476.

depends/hosts/linux.mk

@@ -17,7 +17,7 @@ linux_release_CXXFLAGS=$(linux_release_CFLAGS)
 linux_debug_CFLAGS=-O1
 linux_debug_CXXFLAGS=$(linux_debug_CFLAGS)
 
-linux_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_ENABLE_ASSERTIONS=1
+linux_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_ENABLE_DEBUG_MODE=1
 
 ifeq (86,$(findstring 86,$(build_arch)))
 i686_linux_CC=gcc -m32