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

Change output condition in `_pp`

Merge pull request #5182 from shuujii/use-local-variables-instead-of-top-level-constants-in-boxing.rb

Use local variables instead of top level constants in `boxing.rb` [ci skip]

* Output even with `rake -v` (to help debugging)
* Not output when `rake -s`

Refine build log for generated files

Add brief explanation about `bin/mruby`; ref #5157 [ci skip]

* Output `GEN` log for generated files
* `MRBC` log is outputted one for each `mrbc` execution

#### Before this patch:

```console
CC    src/array.c -> build/host/src/array.o
(snip)
GEN   mrblib/*.rb -> build/host/mrblib/mrblib.c
      MRBC mrblib/00class.rb
      MRBC mrblib/10error.rb
(snip)
CC    mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
      MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC    mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
      MRBC mrbgems/mruby-socket/test/addrinfo.rb
      MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```

#### After this patch:

```console
GEN   build/presym
GEN   build/presym.inc
CC    src/array.c -> build/host/src/array.o
(snip)
GEN   mrblib/*.rb -> build/host/mrblib/mrblib.c
      MRBC mrblib/00class.rb
           mrblib/10error.rb
(snip)
CC    mrbgems/mruby-time/src/time.c -> build/host/mrbgems/mruby-time/src/time.o
GEN   build/host/mrbgems/mruby-time/gem_init.c
      MRBC mrbgems/mruby-time/mrblib/time.rb
(snip)
CC    mrbgems/mruby-socket/test/sockettest.c -> build/host/mrbgems/mruby-socket/test/sockettest.o
GEN   build/host/mrbgems/mruby-socket/gem_test.c
      MRBC mrbgems/mruby-socket/test/addrinfo.rb
      MRBC mrbgems/mruby-socket/test/basicsocket.rb
(snip)
```

Move global function for build to `lib/mruby/core_ext.rb`

Fix GC mark leaks for `MRB_TT_BREAK`; fix #5168

Because `_pp` is originally defined in `lib/mruby/core_ext.rb`, other global
functions are moved to the file.

Fix size of local variable array in struct dumped file