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.
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).
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).
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)
```
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. e993b83c50
* 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)
```
KOBAYASHI Shuji