Switch to a shorter canonical opcode name for PAC return address signing

The previous name wasn't ever used officially anywhere, and is
long and unwieldy. Switch over to a shorter one suggested by
Pedro Miguel Justo.

Also fix up one leftover instance of an initial tentative
placeholder name.

Author: mstorsjo

docs/build/arm64-exception-handling.md

@@ -313,7 +313,7 @@ The unwind codes are encoded according to the table below. All unwind codes are
 |  | 11101010: Custom stack for `MSFT_OP_CONTEXT` |
 |  | 11101100: Custom stack for `MSFT_OP_CLEAR_UNWOUND_TO_CALL` |
 |  | 1111xxxx: reserved |
-| `pac_sign_return_address` | 11111100: sign the return address in `lr` with `pacibsp` |
+| `pac_sign_lr` | 11111100: sign the return address in `lr` with `pacibsp` |
 
 In instructions with large values covering multiple bytes, the most significant bits are stored first. This design makes it possible to find the total size in bytes of the unwind code by looking up only the first byte of the code. Since each unwind code is exactly mapped to an instruction in a prolog or epilog, you can compute the size of the prolog or epilog. Walk from the sequence start to the end, and use a lookup table or similar device to determine the length of the corresponding opcode.
 
@@ -371,7 +371,7 @@ Step 6: Allocate remaining stack, including local area, `<x29,lr>` pair, and out
 | Step # | Flag values | # of instructions | Opcode | Unwind code |
 |--|--|--|--|--|
 | 0 |  |  | `#intsz = RegI * 8;`<br/>`if (CR==01) #intsz += 8; // lr`<br/>`#fpsz = RegF * 8;`<br/>`if(RegF) #fpsz += 8;`<br/>`#savsz=((#intsz+#fpsz+8*8*H)+0xf)&~0xf)`<br/>`#locsz = #famsz - #savsz` |
-| 1 | **CR** == 11 | 1 | `pacibsp` | `sign_ra` |
+| 1 | **CR** == 11 | 1 | `pacibsp` | `pac_sign_lr` |
 | 2 | 0 < **RegI** <= 10 | **RegI** / 2 +<br/> **RegI** % 2 | `stp x19,x20,[sp,#savsz]!`<br/>`stp x21,x22,[sp,#16]`<br/>`...` | `save_regp_x`<br/>`save_regp`<br/>`...` |
 | 3 | **CR** == 01\* | 1 | `str lr,[sp,#(intsz-8)]`\* | `save_reg` |
 | 4 | 0 < **RegF** <= 7 | (**RegF** + 1) / 2 +<br/>(**RegF** + 1) % 2) | `stp d8,d9,[sp,#intsz]`\*\*<br/>`stp d10,d11,[sp,#(intsz+16)]`<br/>`...`<br/>`str d(8+RegF),[sp,#(intsz+fpsz-8)]` | `save_fregp`<br/>`...`<br/>`save_freg` |