arch: add support for RISC-V 32-bit (riscv32) architecture

arch: add support for RISC-V 32-bit (riscv32) architecture

This enables a riscv32 system to be built with a Buildroot generated
toolchain (gcc >= 7.x, binutils >= 2.30, glibc only).

This requires a custom version of glibc 2.26 from the riscv-glibc
repository. Note that there are no tags in this repository, so the
glibc version just consists of the 40 character commit id string.

Thanks to Fabrice Bellard for pointing me towards the 32-bit glibc
repository and for providing the necessary patch to get it to build.

Signed-off-by: Mark Corbin <mark.corbin@embecosm.com>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>