AArch64: Replace @plt/%gotpcrel in data directives with %pltpcrel %gotpcrel (llvm#155776)

Similar to llvm#132569 for RISC-V, replace the unofficial `@plt` and
`@gotpcrel` relocation specifiers, currently only used by clang
-fexperimental-relative-c++-abi-vtables, with %pltpcrel %gotpcrel. The
syntax is not used in humand-written assembly code, and is not supported
by GNU assembler.

Also replace the recent `@funcinit` with `%funcinit(x)`.

Author: MaskRay

bolt/test/AArch64/skip-non-vfuncptr-reloc-in-relative-vtable.s

@@ -31,5 +31,5 @@ _fake_rtti_data:
 _ZTV3gooE:
 	.word	0
 	.word	_fake_rtti_data-_ZTV3gooE-8
-	.word	foo@PLT-_ZTV3gooE-8
+	.word	%pltpcrel(foo)
 	.size	_ZTV3gooE, 12

lld/test/ELF/aarch64-branch-to-branch.s

@@ -35,14 +35,14 @@ vtable:
 # B2B-NEXT: [[VF:[0-9a-f]{8}]]
 # B2B-RELOC-NEXT: R_AARCH64_PLT32 f3
 # NOB2B-RELOC-NEXT: R_AARCH64_PLT32 f1
-.4byte f1@PLT - vtable
+.4byte %pltpcrel(f1)
 # B2B-SAME: [[VF]]
 # B2B-RELOC-NEXT: R_AARCH64_PLT32 f3+0x4
 # NOB2B-RELOC-NEXT: R_AARCH64_PLT32 f2+0x4
-.4byte f2@PLT - vtable
+.4byte %pltpcrel(f2+4)
 # B2B-SAME: [[VF]]
 # RELOC-NEXT: R_AARCH64_PLT32 f3+0x8
-.4byte f3@PLT - vtable
+.4byte %pltpcrel(f3+8)
 
 .section .text._start,"ax"
 .globl _start

lld/test/ELF/aarch64-feature-bti.s

@@ -290,5 +290,5 @@ func1:
 
 .ifdef RELVTABLE_PLT
 // R_AARCH64_PLT32
-.word funcRelVtable@PLT - .
+.word %pltpcrel(funcRelVtable)
 .endif

lld/test/ELF/aarch64-funcinit64-invalid.s

@@ -5,11 +5,11 @@
 
 .rodata
 # ERR: error: relocation R_AARCH64_FUNCINIT64 cannot be used against local symbol
-.8byte func@FUNCINIT
+.8byte %funcinit(func)
 
 .data
 # ERR: error: relocation R_AARCH64_FUNCINIT64 cannot be used against ifunc symbol 'ifunc'
-.8byte ifunc@FUNCINIT
+.8byte %funcinit(ifunc)
 
 .text
 func:

lld/test/ELF/aarch64-funcinit64.s

@@ -10,7 +10,7 @@
 .data
 # CHECK: R_AARCH64_IRELATIVE [[FOO:[0-9a-f]*]]
 # ERR: relocation R_AARCH64_FUNCINIT64 cannot be used against preemptible symbol 'foo'
-.8byte foo@FUNCINIT
+.8byte %funcinit(foo)
 
 .text
 # CHECK: {{0*}}[[FOO]] {{.*}} foo

lld/test/ELF/aarch64-range-thunk-extension-plt32.s

@@ -25,7 +25,7 @@
   .global _start
   .type _start, %function
 _start:
-  .word callee@PLT - .
+  .word %pltpcrel(callee)
 
   .section .text.2, "ax", %progbits
   .global callee

lld/test/ELF/aarch64-reloc-gotpcrel32.s

@@ -17,11 +17,11 @@ _start:  // PC = 0x303a0
 // bar@GOTPCREL-4 = 0x20390 (got entry for `bar`) - 0x303a8 (.) - 4 = 0xe4fffeff
 // CHECK:      Contents of section .data:
 // CHECK-NEXT:  {{.*}} f0fffeff f0fffeff e4fffeff
-  .word bar@GOTPCREL
-  .word bar@GOTPCREL+4
-  .word bar@GOTPCREL-4
+  .word %gotpcrel(bar)
+  .word %gotpcrel(bar+4)
+  .word %gotpcrel(bar-4)
 
 // WARN: relocation R_AARCH64_GOTPCREL32 out of range: {{.*}} is not in [-2147483648, 2147483647]; references 'baz'
 // WARN: relocation R_AARCH64_GOTPCREL32 out of range: {{.*}} is not in [-2147483648, 2147483647]; references 'baz'
-  .word baz@GOTPCREL+0xffffffff
-  .word baz@GOTPCREL-0xffffffff
+  .word %gotpcrel(baz+0xffffffff)
+  .word %gotpcrel(baz-0xffffffff)

lld/test/ELF/aarch64-reloc-plt32.s

@@ -32,6 +32,6 @@
   .globl _start
   _start:
 .data
-  .word foo@PLT - . + 2149589079
-  .word foo@PLT - . - 2145378212
-  .word foo@PLT - .
+  .word %pltpcrel(foo + 2149589079)
+  .word %pltpcrel(foo - 2145378212)
+  .word %pltpcrel(foo)

lld/test/ELF/aarch64-undefined-weak.s

@@ -37,7 +37,7 @@ _start:
 // R_AARCH64_PREL16
  .hword target - .
 // R_AARCH64_PLT32
- .word target@PLT - .
+ .word %pltpcrel(target)
 
 bl_undefweak2:
  bl undefweak2

llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp

@@ -2671,8 +2671,9 @@ const MCExpr *AArch64AsmPrinter::emitPAuthRelocationAsIRelative(
         MCInstBuilder(AArch64::RET).addReg(AArch64::LR), *STI);
   OutStreamer->popSection();
 
-  return MCSymbolRefExpr::create(IRelativeSym, AArch64::S_FUNCINIT,
-                                 OutStreamer->getContext());
+  return MCSpecifierExpr::create(
+      MCSymbolRefExpr::create(IRelativeSym, OutStreamer->getContext()),
+      AArch64::S_FUNCINIT, OutStreamer->getContext());
 }
 
 const MCExpr *