Fix warnings on CXX/linker flags (#93)

1) When `clang` is used as system compiler, libraries were built without respecting LDFLAGS. For example, this affected LTO flags, if any (and it only affected clang, not gcc).

2) Linker flags are registered as CXX flags, which produces warnings during compilation:
```
clang++: warning: -Wl,-z,noexecstack: 'linker' input unused [-Wunused-command-line-argument]
clang++: warning: -Wl,-znoexecheap: 'linker' input unused [-Wunused-command-line-argument]
clang++: warning: -Wl,-z,relro: 'linker' input unused [-Wunused-command-line-argument]
clang++: warning: -Wl,-z,now: 'linker' input unused [-Wunused-command-line-argument]
```

3) Clang does not support `-Wtrampolines` flag:
```
warning: unknown warning option '-Wtrampolines' [-Wunknown-warning-option]
```

4) No linkers support `noexecheap` anymore. `noexecheap` linker flag was a part of PaX patches to GNU ld, (which were dropped in 2017)[https://www.gentoo.org/support/news-items/2017-08-19-hardened-sources-removal.html]. Now ld/ld.lld/ld.gold don't support it and protection of heap is managed by NX bit. Therefore every compiler produces this warning:
```
ld.lld: warning: unknown -z value: noexecheap
```

Change-Id: I2334a4d4c745df2abc12d543616ca179f85c3575

Signed-off-by: Galantsev, Dmitrii <dmitrii.galantsev@amd.com>
Co-authored-by: Sv. Lockal <lockalsash@gmail.com>

cmake_modules/help_package.cmake

@@ -56,8 +56,16 @@ function(generic_package)
         ## Security breach mitigation flags
         set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DFORTIFY_SOURCE=2 -fstack-protector-all -Wcast-align" PARENT_SCOPE)
         ## More security breach mitigation flags
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wl,-z,noexecstack -Wl,-znoexecheap -Wl,-z,relro" PARENT_SCOPE)
-        set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wtrampolines -Wl,-z,now" PARENT_SCOPE)
+        set(HARDENING_LDFLAGS
+            "${HARDENING_LDFLAGS} -Wl,-z,noexecstack -Wl,-z,relro -Wl,-z,now")
+        set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${HARDENING_LDFLAGS}" PARENT_SCOPE)
+        set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${HARDENING_LDFLAGS}" PARENT_SCOPE)
+
+        include(CheckCXXCompilerFlag)
+        check_cxx_compiler_flag("-Wtrampolines" CXX_SUPPORTS_WTRAMPOLINES)
+        if(CXX_SUPPORTS_WTRAMPOLINES)
+            set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wtrampolines" PARENT_SCOPE)
+        endif()
     endif()
 
     # Clang does not set the build-id