Currently, the size of writing in heap by write_irep_record() is
bigger than The size that is calculated by get_irep_record_size.

Therefore, irep is dumped over the size of allocating memory when we
execute dump_irep().

Fix incorrect regexp creation in `tasks/presym.rake`

Fix build failure; ref #5194

Fix the following two issues:

1. Compile failure when external gem is specified.
   * https://github.com/mruby/mruby/pull/5194#issuecomment-734303442
2. When there are multiple build targets, presym parsing sometimes fails or
   output result of presym is broken.
   * https://github.com/mruby/mruby/pull/5194#issuecomment-734322142

The root cause of 2 is unknown, but it seems to occur when presym parsing is
performed in parallel, therefore I change them so that they are not run in
parallel.

Add `tools/*` to target directory for presym parsing

Create presym files for each build target

Previously, presym files were always created in `build/{presym,presym.inc}`.
However, this constraint is inconvenient because it is common to use
multiple build configurations and build targets in a single mruby tree.
Therefore, change to create presym file for each build target.

Fix loop condition in `mrb_sym_all_symbols`

Jump target address is `operand (16bit)` + `address of next instruction`.

In addition, `ilen` was made `uint32_t` so that `iseq` length limitation
of 65536 is removed. Only jump target address should be within signed
16bit (-32768 .. 32767).

There's no efficiency difference since `cdump` is implemented.

Allow compiler name in build log to be customized

For example, in the case of the C++ compiler, it is output as
`CXX  build/host/src/gc.cxx -> build/host/src/gc.cxx.o` (FYI, in the case
of `enable_cxx_abi`, it outputs `CC ...` because the option to compile as
C++ is added to C compiler).

Fix C source compilation with `MRB_USE_ALL_SYMBOLS`; ref #5187

However, compiling by `mrbc` fails with another issue (#5116).

Optimize `presym_find`

Also added `no-float.rb` target in `build_config`.

Chang to compare string length first.

### Benchmark

#### Code

* https://github.com/shuujii/mruby-presym_find-benchmark

#### Result

```console
Previous: 10.240772M i/s (25M times in 2.441222s)
     New: 16.412985M i/s (25M times in 1.523184s)
```

Fix #5177 by setting MRUBY_CONFIG early

Avoid warnings from clang's "-Wembedded-directive"

If it gives clang-11.0 `-Wembedded-directive`, a warning will be reported in `include/mruby/boxing_nan.h`.

```
include/mruby/boxing_nan.h:48:2: warning: embedding a directive within macro arguments has undefined behavior [-Wembedded-directive]
include/mruby/boxing_nan.h:52:2: warning: embedding a directive within macro arguments has undefined behavior [-Wembedded-directive]
```

The cause of this is #5117.
ref. e993b83c

Fix unintended variable shadowing

Raise an exception instead of exiting when `mrbc` fails