Reject known failing MemSan build

[gilles-peskine-arm]

Steps to reproduce (on mbedtls-2.7.0 or 2.8):

scripts/config.pl full
CC=clang cmake -D CMAKE_BUILD_TYPE:String=MemSan .
make
make test

Several tests fail. In fact we know about this — all.sh says

scripts/config.pl unset MBEDTLS_AESNI_C # memsan doesn't grok asm

We should reject the configuration at the cmake or make step. Doing it in cmake may be easier, but doing it at the make step (ideally through a #warning directive so that it's independent of the build system) would have the advantage that it would work even if config.h is changed after running cmake or when using other build systems.

[mpg]

This should be fairly easy to do at the source level:

#if defined(__has_feature)
#if __has_feature(memory_sanitizer)
#warning "This function doesn't work with Memory Sanitizer"
#endif
#endif

We could also try to make the tests actually pass using __attribute__((no_sanitize("memory"))) on some functions (but I'm not sure if it still marks memory written by those functions as initialised) or the sanitizer special case list, though last time I tried the latter approach I couldn't get anywhere.

[gilles-peskine-arm]

Fixed in #1560