Documentation: x86: convert entry_64.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/x86/entry_64.rst

@@ -1,3 +1,9 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+Kernel Entries
+==============
+
 This file documents some of the kernel entries in
 arch/x86/entry/entry_64.S.  A lot of this explanation is adapted from
 an email from Ingo Molnar:
@@ -59,7 +65,7 @@ Now, there's a secondary complication: there's a cheap way to test
 which mode the CPU is in and an expensive way.
 
 The cheap way is to pick this info off the entry frame on the kernel
-stack, from the CS of the ptregs area of the kernel stack:
+stack, from the CS of the ptregs area of the kernel stack::
 
 	xorl %ebx,%ebx
 	testl $3,CS+8(%rsp)
@@ -67,7 +73,7 @@ stack, from the CS of the ptregs area of the kernel stack:
 	SWAPGS
 
 The expensive (paranoid) way is to read back the MSR_GS_BASE value
-(which is what SWAPGS modifies):
+(which is what SWAPGS modifies)::
 
 	movl $1,%ebx
 	movl $MSR_GS_BASE,%ecx
@@ -76,7 +82,7 @@ The expensive (paranoid) way is to read back the MSR_GS_BASE value
 	js 1f   /* negative -> in kernel */
 	SWAPGS
 	xorl %ebx,%ebx
-1:	ret
+  1:	ret
 
 If we are at an interrupt or user-trap/gate-alike boundary then we can
 use the faster check: the stack will be a reliable indicator of

Documentation/x86/index.rst

@@ -12,3 +12,4 @@ x86-specific Documentation
    topology
    exception-tables
    kernel-stacks
+   entry_64