[hwasan] Remove allocator interceptors.

Summary:
When building without COMPILER_RT_HWASAN_WITH_INTERCEPTORS, skip
interceptors for malloc/free/etc and only export their versions with
__sanitizer_ prefix.

Also remove a hack in mallinfo() interceptor that does not apply to
hwasan.

Reviewers: kcc

Subscribers: kubamracek, krytarowski, llvm-commits

Differential Revision: https://reviews.llvm.org/D51711

llvm-svn: 341598