cmd/internal/obj/arm64: summarize the Go assembly syntax and the GNU syntax mapping rules

The patch rewrites the content of doc.go file. The file describes some
general rules of the mapping between Go assembly syntax and GNU syntax.
And it gives some Go assembly examples and corresponding GNU assembly
examples.

The patch changes the doc.go to use standard doc comment format so that
the link https://golang.org/cmd/internal/obj/arm64/ can display it.

Assembly document framework is mainly contributed by Eric Fang <Eric.Fang@arm.com>
Documentation work is contributed by Eric Fang and Fannie Zhang <Fannie.Zhang@arm.com>

Change-Id: I8b3f6d6c6b91afdc2c44602e8f796beea905085e
Reviewed-on: https://go-review.googlesource.com/102055Reviewed-by: Rob Pike <r@golang.org>

doc/asm.html

@@ -738,6 +738,13 @@ The other codes are <code>-&gt;</code> (arithmetic right shift),
 The ARM64 port is in an experimental state.
 </p>
 
+<p>
+<code>R18</code> is the "platform register", reserved on the Apple platform.
+<code>R27</code> and <code>R28</code> are reserved by the compiler and linker.
+<code>R29</code> is the frame pointer.
+<code>R30</code> is the link register.
+</p>
+
 <p>
 Instruction modifiers are appended to the instruction following a period.
 The only modifiers are <code>P</code> (postincrement) and <code>W</code>
@@ -752,11 +759,61 @@ Addressing modes:
 <ul>
 
 <li>
-<code>(R5, R6)</code>: Register pair for <code>LDP</code>/<code>STP</code>.
+<code>R0-&gt;16</code>
+<br>
+<code>R0&gt;&gt;16</code>
+<br>
+<code>R0&lt;&lt;16</code>
+<br>
+<code>R0@&gt;16</code>:
+These are the same as on the 32-bit ARM.
+</li>
+
+<li>
+<code>$(8&lt;&lt;12)</code>:
+Left shift the immediate value <code>8</code> by <code>12</code> bits.
+</li>
+
+<li>
+<code>8(R0)</code>:
+Add the value of <code>R0</code> and <code>8</code>.
+</li>
+
+<li>
+<code>(R2)(R0)</code>:
+The location at <code>R0</code> plus <code>R2</code>.
+</li>
+
+<li>
+<code>R0.UXTB</code>
+<br>
+<code>R0.UXTB&lt;&lt;imm</code>:
+<code>UXTB</code>: extract an 8-bit value from the low-order bits of <code>R0</code> and zero-extend it to the size of <code>R0</code>.
+<code>R0.UXTB&lt;&lt;imm</code>: left shift the result of <code>R0.UXTB</code> by <code>imm</code> bits.
+The <code>imm</code> value can be 0, 1, 2, 3, or 4.
+The other extensions include <code>UXTH</code> (16-bit), <code>UXTW</code> (32-bit), and <code>UXTX</code> (64-bit).
+</li>
+
+<li>
+<code>R0.SXTB</code>
+<br>
+<code>R0.SXTB&lt;&lt;imm</code>:
+<code>SXTB</code>: extract an 8-bit value from the low-order bits of <code>R0</code> and sign-extend it to the size of <code>R0</code>.
+<code>R0.SXTB&lt;&lt;imm</code>: left shift the result of <code>R0.SXTB</code> by <code>imm</code> bits.
+The <code>imm</code> value can be 0, 1, 2, 3, or 4.
+The other extensions include <code>SXTH</code> (16-bit), <code>SXTW</code> (32-bit), and <code>SXTX</code> (64-bit).
+</li>
+
+<li>
+<code>(R5, R6)</code>: Register pair for <code>LDAXP</code>/<code>LDP</code>/<code>LDXP</code>/<code>STLXP</code>/<code>STP</code>/<code>STP</code>.
 </li>
 
 </ul>
 
+<p>
+Reference: <a href="/pkg/cmd/internal/obj/arm64">Go ARM64 Assembly Instructions Reference Manual</a>
+</p>
+
 <h3 id="ppc64">64-bit PowerPC, a.k.a. ppc64</h3>
 
 <p>