build: build x86_64 Linux Boost with -fcf-protection=full

This is required so that we can test for control flow in our ELF
security checks. Otherwise test_bitcoin will fail, as it has a main that
is provided by Boost, and wont have been built with
-fcf-protection=full.

While here simplify to using -fcf-protection for all x86_64 builds.

depends/packages/boost.mk

@@ -26,8 +26,7 @@ $(package)_config_libraries=filesystem,system,test
 $(package)_cxxflags+=-std=c++17
 $(package)_cxxflags_linux=-fPIC
 $(package)_cxxflags_android=-fPIC
-$(package)_cxxflags_x86_64_darwin=-fcf-protection=full
-$(package)_cxxflags_mingw32=-fcf-protection=full
+$(package)_cxxflags_x86_64=-fcf-protection=full
 endef
 
 define $(package)_preprocess_cmds