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.

targets/riscv32.json

@@ -1,12 +1,12 @@
 {
 	"inherits": ["riscv"],
 	"llvm-target": "riscv32-unknown-none",
+	"target-abi": "ilp32",
 	"build-tags": ["tinygo.riscv32"],
 	"scheduler": "tasks",
 	"default-stack-size": 2048,
 	"cflags": [
-		"-march=rv32imac",
-		"-mabi=ilp32"
+		"-march=rv32imac"
 	],
 	"ldflags": [
 		"-melf32lriscv"

targets/riscv64.json

@@ -1,10 +1,10 @@
 {
 	"inherits": ["riscv"],
 	"llvm-target": "riscv64-unknown-none",
+	"target-abi": "lp64",
 	"build-tags": ["tinygo.riscv64"],
 	"cflags": [
-		"-march=rv64gc",
-		"-mabi=lp64"
+		"-march=rv64gc"
 	],
 	"ldflags": [
 		"-melf64lriscv"