Merge bitcoin#29781: depends: add new LLVM debug macro

5efebc0 depends: add the new LLVM debug macro (fanquake)

Pull request description:

  `LIBCXX_HARDENING_MODE` is the new macro, the previous one was removed in LLVM 18.

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

  Required before google/oss-fuzz#11725 will do anything (with the bump to 18.x).

  Seems reasonable to do now that almost all our test infra is using LLVM 18.

ACKs for top commit:
  theuni:
    ACK 5efebc0

Tree-SHA512: 43078eeb5940c55ef4f95c72682f8a372dcd3eb97956b3114149c16d9f59b067a999b2aab7f34ffb57eab191524514408e2bba154ff4a6ea0cd6ec4d119c5d18

depends/hosts/linux.mk

@@ -13,7 +13,11 @@ linux_release_CXXFLAGS=$(linux_release_CFLAGS)
 linux_debug_CFLAGS=-O1 -g
 linux_debug_CXXFLAGS=$(linux_debug_CFLAGS)
 
-linux_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -D_LIBCPP_ENABLE_DEBUG_MODE=1
+# https://gcc.gnu.org/onlinedocs/libstdc++/manual/debug_mode.html
+linux_debug_CPPFLAGS=-D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC
+
+# https://libcxx.llvm.org/Hardening.html
+linux_debug_CPPFLAGS+=-D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_DEBUG
 
 ifeq (86,$(findstring 86,$(build_arch)))
 i686_linux_CC=gcc -m32