cpu/fe310: set back newlib as default libc

[aabadie]

Contribution description

This PR fixes a regression introduced by the combination of #15859 and #15957. Initially newlib was used as default libc on fe310 and the regression forces the use of picolibc.

For users with local toolchains providing newlib by default, this regression is a breaking change. Note that in recent Ubuntu versions, riscv64-unknown-elf is available in the package manager but it doesn't come with newlib and the package manager only provides picolibc. So it's not possible to use the riscv toolchain in the package manager of Ubuntu unless FEATURES_REQUIRED=picolibc is added to the build command. So this is not optimal but there's no good solution for all setup at the moment (until we have a way to automatically detect and use the available libc).

Testing procedure

• A green Murdock (Murdock provides both newlib and picolibc)
• Use a prebuilt toolchain from https://github.com/xpack-dev-tools/riscv-none-embed-gcc-xpack with picolibc installed and build a firmware for hifive1b:

this PR

$ make BOARD=hifive1b -C examples/hello-world --no-print-directory 
Building application "hello-world" for "hifive1b" with MCU "fe310".

"make" -C /work/riot/RIOT/boards/hifive1b
"make" -C /work/riot/RIOT/core
"make" -C /work/riot/RIOT/cpu/fe310
"make" -C /work/riot/RIOT/cpu/fe310/periph
"make" -C /work/riot/RIOT/cpu/fe310/vendor
"make" -C /work/riot/RIOT/cpu/riscv_common
"make" -C /work/riot/RIOT/cpu/riscv_common/periph
"make" -C /work/riot/RIOT/drivers
"make" -C /work/riot/RIOT/drivers/periph_common
"make" -C /work/riot/RIOT/sys
"make" -C /work/riot/RIOT/sys/auto_init
"make" -C /work/riot/RIOT/sys/malloc_thread_safe
"make" -C /work/riot/RIOT/sys/newlib_syscalls_default
"make" -C /work/riot/RIOT/sys/stdio_uart
   text	   data	    bss	    dec	    hex	filename
   7790	    108	   2248	  10146	   27a2	/work/riot/RIOT/examples/hello-world/bin/hifive1b/hello-world.elf

master

$ make BOARD=hifive1b -C examples/hello-world --no-print-directory 
Building application "hello-world" for "hifive1b" with MCU "fe310".

"make" -C /work/riot/RIOT/boards/hifive1b
"make" -C /work/riot/RIOT/core
"make" -C /work/riot/RIOT/cpu/fe310
"make" -C /work/riot/RIOT/cpu/fe310/periph
"make" -C /work/riot/RIOT/cpu/fe310/vendor
"make" -C /work/riot/RIOT/cpu/riscv_common
"make" -C /work/riot/RIOT/cpu/riscv_common/periph
"make" -C /work/riot/RIOT/drivers
"make" -C /work/riot/RIOT/drivers/periph_common
"make" -C /work/riot/RIOT/sys
"make" -C /work/riot/RIOT/sys/auto_init
"make" -C /work/riot/RIOT/sys/malloc_thread_safe
"make" -C /work/riot/RIOT/sys/picolibc_syscalls_default
/work/riot/RIOT/sys/picolibc_syscalls_default/syscalls.c:236:5: error: implicit declaration of function 'FDEV_SETUP_STREAM' [-Werror=implicit-function-declaration]
  236 |     FDEV_SETUP_STREAM(picolibc_put, picolibc_get, picolibc_flush, _FDEV_SETUP_RW);
      |     ^~~~~~~~~~~~~~~~~
/work/riot/RIOT/sys/picolibc_syscalls_default/syscalls.c:236:67: error: '_FDEV_SETUP_RW' undeclared here (not in a function)
  236 |     FDEV_SETUP_STREAM(picolibc_put, picolibc_get, picolibc_flush, _FDEV_SETUP_RW);
      |                                                                   ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[3]: *** [/work/riot/RIOT/Makefile.base:107: /work/riot/RIOT/examples/hello-world/bin/hifive1b/picolibc_syscalls_default/syscalls.o] Error 1
make[2]: *** [/work/riot/RIOT/Makefile.base:30: ALL--/work/riot/RIOT/sys/picolibc_syscalls_default] Error 2
make[1]: *** [/work/riot/RIOT/Makefile.base:30: ALL--/work/riot/RIOT/sys] Error 2
make: *** [/work/riot/RIOT/examples/hello-world/../../Makefile.include:618: application_hello-world.module] Error 2

Issues/PRs references

#15859 and #15957

[maribu]

ACK