[code-reflection] RFR: [hat] Enable padding in the codegen as an option for the iFace types and Schemas

[Juan Fumero]

This PR extends the C99 code generator to allow the generation of padding if this option is specified in the Hat Data Type Schema. 

For instance: 


Schema<F32Array> schema = Schema.of(F32Array.class, f32Array->f32Array
            .arrayLen("length").pad(12).array("array"));


Generates this C99 struct:


typedef struct F32Array_s{
    int length;
    char pad$ZArmN[12];   // pad$ + 5 random letter to avoid accidental collision with other members of the buffer type
    float array[1];
}F32Array_t;


This change is required to enable efficient local memory: See https://github.com/openjdk/babylon/pull/531

-------------

Commit messages:
 - [hat] Disable padding for F32Array
 - [hat] Enable padding for iFace schemas in the C99 codegen

Changes: https://git.openjdk.org/babylon/pull/534/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=534&range=00
  Stats: 29 lines in 2 files changed: 27 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/babylon/pull/534.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/534/head:pull/534

PR: https://git.openjdk.org/babylon/pull/534