Revert "toolchain/toolchain-buildroot: enable uclibc for riscv64"

Revert "toolchain/toolchain-buildroot: enable uclibc for riscv64"

This reverts commit bd9810e176273914eca1208bcba23f0de9e446b3. Indeed,
while uClibc-ng has support for RISC-V 64-bit, this support lacks the
__riscv_flush_icache() function call, which is used by some GCC
builtins used for example in libffi.

Due to this missing __riscv_flush_icache(), anything that links
against libffi fails to build:

/home/test/autobuild/run/instance-0/output-1/host/bin/riscv64-buildroot-linux-uclibc-gcc  -o gobject/gobject-query gobject/gobject-query.p/gobject-query.c.o -Wl,--as-needed -Wl,--no-undefined -Wl,-O1 -Wl,--start-group glib/libglib-2.0.so.0.6400.4 gobject/libgobject-2.0.so.0.6400.4 -Wl,--end-group -pthread '-Wl,-rpath,$ORIGIN/../glib:$ORIGIN/' -Wl,-rpath-link,/home/test/autobuild/run/instance-0/output-1/build/libglib2-2.64.4/build/glib -Wl,-rpath-link,/home/test/autobuild/run/instance-0/output-1/build/libglib2-2.64.4/build/gobject
/home/test/autobuild/run/instance-0/output-1/host/lib/gcc/riscv64-buildroot-linux-uclibc/9.3.0/../../../../riscv64-buildroot-linux-uclibc/bin/ld: /home/test/autobuild/run/instance-0/output-1/host/riscv64-buildroot-linux-uclibc/sysroot/usr/lib64/libffi.so.7: undefined reference to `__riscv_flush_icache'
collect2: error: ld returned 1 exit status

Note that this commit means that
support/config-fragments/autobuild/br-riscv64-full-internal.config
will be back to using glibc as the C library, but that is OK, until
uClibc-ng is fixed to implemented __riscv_flush_icache().

This uClibc-ng issue has been reported upstream at
https://mailman.uclibc-ng.org/pipermail/devel/2020-August/002022.html.

Fixes:

  http://autobuild.buildroot.net/results/ec1185ad1fd8863a3990143a0af2ace987761a27/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>