x86: Uses rbx for the initial vcoreid (XCC)

Using rax prevents us from using SW contexts.

Rebuild your toolchain.  You don't need to make clean.

kern/arch/x86/process64.c

diff --git a/kern/arch/x86/process64.c b/kern/arch/x86/process64.c
index 3e6dc38..b942722 100644
--- a/kern/arch/x86/process64.c
+++ b/kern/arch/x86/process64.c
@@ -93,7 +93,7 @@
 	tf->tf_rip = entryp;
 	/* Coupled closely with user's entry.S.  id is the vcoreid, which entry.S
 	 * uses to determine what to do.  vcoreid == 0 is the main core/context. */
-	tf->tf_rax = vcoreid;
+	tf->tf_rbx = vcoreid;
 	tf->tf_fsbase = tls_desc;
 	proc_secure_ctx(ctx);
 }

user/parlib/include/x86/vcore32.h

diff --git a/user/parlib/include/x86/vcore32.h b/user/parlib/include/x86/vcore32.h
index d3a3e3d..d133273 100644
--- a/user/parlib/include/x86/vcore32.h
+++ b/user/parlib/include/x86/vcore32.h
@@ -316,7 +316,7 @@
 // this is how we get our thread id on entry.
 #define __vcore_id_on_entry \
 ({ \
-	register int temp asm ("eax"); \
+	register int temp asm ("ebx"); \
 	temp; \
 })
 

user/parlib/include/x86/vcore64.h

diff --git a/user/parlib/include/x86/vcore64.h b/user/parlib/include/x86/vcore64.h
index 9ae612d..d746392 100644
--- a/user/parlib/include/x86/vcore64.h
+++ b/user/parlib/include/x86/vcore64.h
@@ -355,7 +355,7 @@
 // this is how we get our thread id on entry.
 #define __vcore_id_on_entry \
 ({ \
-	register int temp asm ("rax"); \
+	register int temp asm ("rbx"); \
 	temp; \
 })