Add build script to extract a list of registered syscalls

The list is used in cargo-build-bpf to check generated .so modules for
undefined symbols that are not known run-time syscalls.

programs/bpf_loader/Cargo.toml

@@ -9,6 +9,9 @@ homepage = "https://solana.com/"
 documentation = "https://docs.rs/solana-bpf-loader-program"
 edition = "2018"
 
+[build-dependencies]
+regex = "1.5.4"
+
 [dependencies]
 bincode = "1.3.1"
 byteorder = "1.3.4"