Bump bpf-tools to v1.20

- Fix C standard library to include only reentrant versions of functions
- Use a version of solana_rbpf with fixed relocations handling
- Add a test of using C standard library functions in on-chain program

programs/bpf/c/src/stdlib/stdlib.c

@@ -0,0 +1,20 @@
+/**
+ * @brief Example C based BPF program that uses C standard
+ * library functions.  The test fails if the C standard
+ * library is not available.
+ *
+ * Only re-entrant versions of standard C library functions are
+ * available.  We have to use the _reent structure and pass a pointer
+ * to it as a parameter to a re-entrant function.  In this test
+ * _atoi_r is a re-entrant version of the standard atoi function.
+ */
+#include <sol/assert.h>
+#include <stdlib.h>
+
+extern uint64_t entrypoint(const uint8_t *input) {
+  struct _reent reent;
+  int value = _atoi_r(&reent, "137");
+  sol_assert(value == 137);
+
+  return SUCCESS;
+}

programs/bpf/tests/programs.rs

@@ -441,6 +441,7 @@ fn test_program_bpf_sanity() {
             ("sanity++", true),
             ("secp256k1_recover", true),
             ("sha", true),
+            ("stdlib", true),
             ("struct_pass", true),
             ("struct_ret", true),
         ]);