Cannot use `SAN` CMake option in app builds

[letmaik]

The CCF CMake helpers installed in /opt/ccf/cmake interpret -DSAN=ON and enable SAN-related compile flags. This fails with:

clang: error: no such file or directory: '/opt/ccf/src/ubsan.suppressions'

If the SAN option is something that is part of the public CCF CMake API, then probably the missing file should be installed (or some variant of it). If not, then a less "global" variable name should be used.

[letmaik]

I note also that implementing an app-specific CMake SAN option is currently tricky because of the following lines in add_ccf_app:

CCF/cmake/ccf_app.cmake

Lines 188 to 190 in c856048

	if(NOT SAN)
	target_link_options(${virt_name} PRIVATE LINKER:--no-undefined)
	endif()

[letmaik]

As a temporary hack, I copied ubsan.suppressions to the right place, but because CCF itself (cchost) was built without SAN enabled, it fails at runtime:

terminating with uncaught exception of type std::logic_error: Could not load virtual enclave: ./libfoo.virtual.so: undefined symbol: __ubsan_vptr_type_cache

It seems like the only option is to build CCF from source in order to use SAN for apps.

[letmaik]

Summary of steps to use SAN in apps as of now:

1. Build CCF from source with -DSAN=ON
2. Introduce new option in app's CMake, e.g. -DENABLE_SAN=ON
3. Write CMake code similar to CCF's code to add the right compile and link options to targets if ENABLE_SAN is on.

For 3, this can be used as starting point:

CCF/cmake/tools.cmake

Lines 4 to 19 in 5140e0f

	function(add_san name)
	if(SAN)
	target_compile_options(
	${name}
	PRIVATE -fsanitize=undefined,address -fno-omit-frame-pointer
	-fno-sanitize-recover=all -fno-sanitize=function
	-fsanitize-blacklist=${CCF_DIR}/src/ubsan.suppressions
	)
	target_link_libraries(
	${name}
	PRIVATE -fsanitize=undefined,address -fno-omit-frame-pointer
	-fno-sanitize-recover=all -fno-sanitize=function
	-fsanitize-blacklist=${CCF_DIR}/src/ubsan.suppressions
	)
	endif()
	endfunction()