Prepare jit source for clang-tidy

Prepare jit source for clang-tidy

In templates, clang-tidy will replace 0 with nullptr, but this is illegal
for primitive types. This change replaces 0 in two templated functions
with ZERO, which is defined to be 0 to avoid this issue.

Additioanlly, we need to conditionally define some macros for running
under __clang__ so they are found by clang-tidy.

Commit migrated from https://github.com/dotnet/coreclr/commit/ab7d6a8df73d3d89210a778338feaa9fedf4146a