cpu/fe310: blacklist picolibc on CI

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.

cpu/fe310/Kconfig

@@ -8,7 +8,7 @@
 config CPU_ARCH_RISCV
     bool
     select HAS_ARCH_RISCV
-    select HAS_PICOLIBC
+    select HAS_PICOLIBC if '$(RIOT_CI_BUILD)' != '1'
 
 config CPU_CORE_RV32M
     bool

cpu/fe310/Makefile.features

@@ -7,5 +7,9 @@ FEATURES_PROVIDED += periph_gpio periph_gpio_irq
 FEATURES_PROVIDED += periph_plic
 FEATURES_PROVIDED += periph_pm
 FEATURES_PROVIDED += periph_wdt
-FEATURES_PROVIDED += picolibc
 FEATURES_PROVIDED += ssp
+
+# RISC-V toolchain on CI does not work properly with picolibc yet
+ifeq (,$(RIOT_CI_BUILD))
+  FEATURES_PROVIDED += picolibc
+endif