Now that we use LLVM 9, RISC-V support in LLVM has far fewer bugs and we
can avoid the GNU toolchain.
* replace GNU linker with lld
* replace GCC with clang
Additionally, RISC-V was promoted to stable so it can be enabled by
default in CI.
The default Go version is 1.12. Because Go 1.13 introduced language
changes (improved numeric constants), TinyGo compiled with Go 1.12
cannot handle Go 1.13 code such as the Go 1.13 standard library.
Use Go 1.13 to build TinyGo on Azure Pipelines to fix this.
