[dev.boringcrypto] crypto/internal/boring: fall back to standard crypto when using...

[dev.boringcrypto] crypto/internal/boring: fall back to standard crypto when using -msan

The syso is not compiled with -fsanitize=memory, so don't try to use it.
Otherwise the first time it calls out to memcmp, memcmp complains
that it is being asked to compare uninitialized memory.

Change-Id: I85ab707cfbe64eded8e110d4d6b40d1b75f50541
Reviewed-on: https://go-review.googlesource.com/63916
Reviewed-by: Adam Langley <agl@golang.org>