lib: c: add pthread implementation

lib/libc/pthread/sys/x86_64/pthread_unmap_exit.S

@@ -0,0 +1,35 @@
+.code64
+
+#include "mango/syscall.h"
+
+.global __pthread_unmap_exit
+.type __pthread_unmap_exit, @function
+
+/*
+  %rdi = (kern_handle_t)address_space
+  %rsi = (void *)unmap_base
+  %rdx = (size_t)unmap_length
+ */
+__pthread_unmap_exit:
+	push %rbp
+	mov %rsp, %rbp
+
+	/* save the address space handle for later */
+	mov %rdi, %rbx
+
+	/* first, unmap the specified region */
+	mov $SYS_ADDRESS_SPACE_UNMAP, %rax
+	/* args are already in the correct registers */
+	syscall
+
+	/* next, close the handle to the address space */
+	mov $SYS_THREAD_EXIT, %rax
+	mov %rbx, %rdi
+	syscall
+
+	/* finally, stop the current thread */
+	mov $SYS_THREAD_EXIT, %rax
+	syscall
+
+	/* unreachable */
+	ret