Riscv64gc support in the build scripts

This architecture is different from x86/aarch because its rust triplet (riscv64gc-...) differs from gcc/llvm/binutils triplet (more general riscv64-...). Therefore the build scripts have to bridge that mismatch.

This introduces new build-time environment variable GNU_TARGET which corresponds to a target used by C world utilities whereas existing TARGET now describes rust target.

config/riscv64gc/minimal.toml

@@ -0,0 +1,14 @@
+# Minimal configuration
+
+include = ["../minimal.toml"]
+
+# Override the default settings here
+
+# General settings
+[general]
+# Filesystem size in MiB
+filesystem_size = 128
+
+# Package settings
+[packages]
+# example = {}