Implement arithmetic negation

Rakefile

@@ -6,7 +6,6 @@
 require 'open3'
 require 'rake/clean'
 
-CROSS_GCC = '../eugenios/build/rootfs/bin/riscv32-unknown-linux-gnu-gcc'
 SYSROOT = '../eugenios/build/sysroot'
 QEMU = 'qemu-riscv32'
 STAGES = Dir.glob('boot/stage*.elna').collect { |stage| File.basename stage, '.elna' }.sort
@@ -16,6 +15,10 @@ CLEAN.include 'build/boot', 'build/valid'
 directory 'build/boot'
 directory 'build/valid'
 
+def compile(input, output)
+  sh ENV.fetch('CC', 'gcc'), '-nostdlib', '-o', output, input
+end
+
 task default: :boot
 
 desc 'Final stage'
@@ -47,7 +50,7 @@ end
 rule /^build\/[[:alpha:]]+\/stage[[:digit:]]+$/ => ->(match) {
   "#{match}.s"
 } do |t|
-  sh CROSS_GCC, '-nostdlib', '-o', t.name, *t.prerequisites
+  compile(*t.prerequisites, t.name)
 end
 
 STAGES.each do |stage|
@@ -91,5 +94,5 @@ end
 file 'build/boot/stage1' => ['build/boot', 'boot/stage1.s'] do |t|
   source = t.prerequisites.select { |prerequisite| prerequisite.end_with? '.s' }
 
-  sh CROSS_GCC, '-nostdlib', '-o', t.name, *source
+  compile(*source, t.name)
 end