Tweaks to allow Compilation on both Scala 2.13 and 2.12

[pete-restall]

Hello,

I needed support for Scala 2.13 so I forked VexRiscV and added it as a cross target. My fork certainly works for my use-case and I thought I'd contribute back as it may be of use to yourselves and others that wish to integrate.

Basically, in the absence of packages, it allows me to build via SBT as a sub-module like this:

ThisBuild / scalaVersion := "2.13.18"

// ...

val spinalVersion = "1.13.0"

lazy val root = (project in file("."))
	.settings(
		// ...
		libraryDependencies ++= Seq(
			"org.scalatest" %% "scalatest" % "3.2.17" % Test,
			"com.github.spinalhdl" %% "spinalhdl-core" % spinalVersion,
			"com.github.spinalhdl" %% "spinalhdl-lib" % spinalVersion,
			compilerPlugin("com.github.spinalhdl" %% "spinalhdl-idsl-plugin" % spinalVersion)))

	.dependsOn(vexriscv)

lazy val vexriscv = RootProject(uri("https://github.com/pete-restall/fork-VexRiscv.git#3261ef67efd64f9d2c21ed9b55f0ea3a1bb62d58"))

I'm happy to tweak it if you need me to.

Cheers.

[pete-restall]

I think we're good now - default version reverted as requested.