Custom heap is BPF only

[jackcmay]

Problem

Custom heaps only apply to BPF programs but its inclusion in entrypoint! means it also shows up when using program-test so programs must feature guard the use of entrypoint! from the outside

Summary of Changes

Add feature guard to the entrypoint! macro to avoid the conflict or doing this:
https://github.com/solana-labs/solana-program-library/blob/487ad2d2d7cb0b8a58f02a156a659e6c8a74f354/examples/rust/logging/src/entrypoint.rs#L3

Mentioned: solana-labs/solana-program-library#829

Fixes #

[codecov]

Codecov Report

Merging #13537 (a5ca268) into v1.4 (2cb9ca5) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##            v1.4   #13537     +/-   ##
========================================
- Coverage   82.2%    82.2%   -0.1%     
========================================
  Files        378      378             
  Lines      90197    90197             
========================================
- Hits       74152    74149      -3     
- Misses     16045    16048      +3     

[joncinque]

This is an even better solution, thanks!