The RISC-V toolchain in riotdocker has issues with picolibc and
will still include newlib headers.
This leads to conflicts like
```
In file included from [01m[Knanostubs.c:22[m[K:
[01m[K/usr/local/picolibc/riscv-none-embed/include/stdio.h:270:23:[m[K [01;31m[Kerror: [m[Kconflicting types for '[01m[K__FILE[m[K'
typedef struct __file [01;31m[K__FILE[m[K;
[01;31m[K^~~~~~[m[K
In file included from [01m[K/opt/gnu-mcu-eclipse/riscv-none-gcc/8.2.0-2.2-20190521-0004/riscv-none-embed/include/reent.h:93[m[K,
from [01m[Knanostubs.c:20[m[K:
[01m[K/opt/gnu-mcu-eclipse/riscv-none-gcc/8.2.0-2.2-20190521-0004/riscv-none-embed/include/sys/reent.h:287:26:[m[K [01;36m[Knote: [m[Kprevious declaration of '[01m[K__FILE[m[K' was here
typedef struct __sFILE [01;36m[K__FILE[m[K;
[01;36m[K^~~~~~[m[K
```
The problem does not occur when installing both the toolchain and picolibc
directly from the Debian / Ubuntu repositories, but CI uses an older Ubuntu
version that does not have those packages yet, so it builds them manually.
Blacklist RISC-V until CI has been updated.
- Add libstdcpp feature to indicate a platform is providing a libstdc++
implementation ready for use
- The existing cpp feature now only indicates a working C++ toolchain without
libstdc++. (E.g. still useful for the Arduino compatibility layer.)
- Added libstdcpp as required feature were needed
- Added some documentation on C++ on RIOT
