riscv: add "target-abi" metadata flag

This flag is necessary in LLVM 15 because it appears that LLVM 15 has
changed the default target ABI from lp64 to lp64d. This results in a
linker failure. Setting the "target-abi" forces the RISC-V backend to
use the intended target ABI.

compiler/compiler_test.go

@@ -73,6 +73,7 @@ func TestCompiler(t *testing.T) {
 			compilerConfig := &Config{
 				Triple:             config.Triple(),
 				Features:           config.Features(),
+				ABI:                config.ABI(),
 				GOOS:               config.GOOS(),
 				GOARCH:             config.GOARCH(),
 				CodeModel:          config.CodeModel(),