Add syscalls to x86-64 linux

.gitignore

@@ -12,3 +12,6 @@ __test__*__.core
 /docs.json
 
 /*.lst
+
+# Ninja build
+.ninja_*

arch/build.ninja

@@ -0,0 +1,9 @@
+rule gas
+  command = gcc -c $in -o $out
+
+rule archive
+  command = ar rcs $out $in
+
+build syscall.o: gas x64/linux/syscall.S
+
+build tanya.a: archive syscall.o

arch/x64/linux/syscall.S

@@ -0,0 +1,65 @@
+/*
+The kernel uses the following registers:
+%rdi, %rsi, %rdx, %r8, %r9, %r10
+
+The number of the syscall is passed in %rax.
+
+A syscall clobbers:
+%rax, %rcx, %r11
+
+The returned value is placed in %rax.
+*/
+	.text
+
+	.globl syscall1
+	.type syscall1, @function
+
+syscall1:
+	movq %rsi, %rax // Syscall number.
+
+	syscall
+
+	ret
+
+
+	.globl syscall2
+	.type syscall2, @function
+
+syscall2:
+	// Store registers.
+	movq %rdi, %r8
+
+	movq %rdx, %rax // Syscall number.
+
+	// Syscall arguments.
+	movq %rsi, %rdi
+	movq %r8, %rsi
+
+	syscall
+
+	// Restore registers.
+	movq %rdi, %rsi
+	movq %r8, %rdi
+
+	ret
+
+
+	.globl syscall3
+	.type syscall3, @function
+
+syscall3:
+	// Store registers.
+	movq %rdi, %r8
+
+	movq %rcx, %rax // Syscall number.
+
+	// Syscall arguments.
+	movq %rdx, %rdi
+	movq %r8, %rdx
+
+	syscall
+
+	// Restore registers.
+	movq %r8, %rdi
+
+	ret