wasm: specify wasi-libc in code, not in the JSON target file

This brings a bit more consistency to libc configuration. It seems
better to me to set the header flags all in the same place, instead of
some in Go code and some in JSON target files (depending on the target).

targets/wasi.json

@@ -6,9 +6,6 @@
 	"goarch":        "arm",
 	"linker":        "wasm-ld",
 	"libc":          "wasi-libc",
-	"cflags": [
-		"--sysroot={root}/lib/wasi-libc/sysroot"
-	],
 	"ldflags": [
 		"--allow-undefined",
 		"--stack-first",

targets/wasm.json

@@ -6,9 +6,6 @@
 	"goarch":        "wasm",
 	"linker":        "wasm-ld",
 	"libc":          "wasi-libc",
-	"cflags": [
-		"--sysroot={root}/lib/wasi-libc/sysroot"
-	],
 	"ldflags": [
 		"--allow-undefined",
 		"--stack-first",