add RISC-V architecture support

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>

config/arch/riscv.in

@@ -0,0 +1,10 @@
+# RISC-V specific config options
+
+## depends on EXPERIMENTAL
+##
+## select ARCH_SUPPORTS_32
+## select ARCH_DEFAULT_32
+## select GCC_REQUIRE_7_or_later
+
+## help The RISC-V architecture, as defined by:
+## help     http://www.riscv.org/

scripts/build/arch/riscv.sh

@@ -0,0 +1,5 @@
+# Compute RISC-V-specific values
+
+CT_DoArchTupleValues() {
+    CT_TARGET_ARCH="riscv${CT_ARCH_BITNESS}"
+}