Add new ASSERT that only activates in debug builds.

This a feature request to make a version of DR_ASSERT that automatically turns void during release builds.

Currently, DR_ASSERT and DR_ASSERT_MSG cannot be easily turned off for release builds. Unlike for <assert.h>, there is no "NDEBUG" version that turns DR asserts off. Therefore, a lot of code, particularly DR extensions, repeatedly resort to defining a new version of ASSERT to resolve the issue:

#ifdef DEBUG
#    define ASSERT(x, msg) DR_ASSERT_MSG(x, msg)
#else
#    define ASSERT(x, msg)
#endif

Add DR_ASSERT_DEBUG that is controllable with respect to the DEBUG def.