support/config-fragments: fix br-riscv{32,64} toolchain fragments

support/config-fragments: fix br-riscv{32,64} toolchain fragments

Since commit aee39cbf27e833fad9962eb99b5655dbadcfee60 ("arch/riscv:
set the default float ABI based on ISA extensions"), RISC-V 32/64 use
the lp32d/lp64d ABIs by default. But our pre-built external toolchains
were built with the LP32/LP64 ABI.

Building with lp32d/lp64d gcc flags, but a toolchain built with the
LP32/LP64 ABI causes a number of failures such as:

  /home/mark/buildroot-test/instance-1/output/host/riscv64-buildroot-linux-gnu/sysroot/usr/include/gnu/stubs.h:11:11: fatal error: gnu/stubs-lp64d.h: No such file or directory

or:

  /home/mark/buildroot-test/instance-1/output/host/opt/ext-toolchain/bin/../lib/gcc/riscv64-buildroot-linux-gnu/7.4.0/../../../../riscv64-buildroot-linux-gnu/bin/ld: /tmp/cc2BTtFE.o: can't link hard-float modules with soft-float modules
  /home/mark/buildroot-test/instance-1/output/host/opt/ext-toolchain/bin/../lib/gcc/riscv64-buildroot-linux-gnu/7.4.0/../../../../riscv64-buildroot-linux-gnu/bin/ld: failed to merge target specific data of file /tmp/cc2BTtFE.o

So let's fix our config fragments to reflect the ABIs those toolchains
were built with.

Fixes:

  http://autobuild.buildroot.net/results/a3959b0613cf561059483abc580b144be4817d1a/ (libsepol)
  http://autobuild.buildroot.net/results/3db50d8a0a913413b2198d6c301419136d2d22a7/ (attr)
  http://autobuild.buildroot.net/results/7780fada05b8440ae3e97618615624a6a2dac03f/ (libusb)
  and many others

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>