[ELF] Support .cfi_signal_frame

glibc/sysdeps/unix/sysv/linux/x86_64/sigaction.c libc.a(sigaction.o) has a CIE
with the augmentation string "zRS". Support 'S' to allow --icf={safe,all}.

lld/ELF/EhFrame.cpp

@@ -188,19 +188,14 @@ uint8_t EhReader::getFdeEncoding() {
   for (char c : aug) {
     if (c == 'R')
       return readByte();
-    if (c == 'z') {
+    if (c == 'z')
       skipLeb128();
-      continue;
-    }
-    if (c == 'P') {
-      skipAugP();
-      continue;
-    }
-    if (c == 'L') {
+    else if (c == 'L')
       readByte();
-      continue;
-    }
-    failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
+    else if (c == 'P')
+      skipAugP();
+    else if (c != 'S')
+      failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
   }
   return DW_EH_PE_absptr;
 }
@@ -216,7 +211,7 @@ bool EhReader::hasLSDA() {
       skipAugP();
     else if (c == 'R')
       readByte();
-    else
+    else if (c != 'S')
       failOn(aug.data(), "unknown .eh_frame augmentation string: " + aug);
   }
   return false;

lld/test/ELF/eh-frame-value-format7.s

@@ -22,7 +22,7 @@
 # CHECK-NEXT:   EntrySize: 0
 # CHECK-NEXT:   SectionData (
 # CHECK-NEXT:     0000: 011B033B 10000000 01000000 30F2FFFF
-# CHECK-NEXT:     0010: 24000000
+# CHECK-NEXT:     0010: 2C000000
 # Header (always 4 bytes): 011B033B
 #    10000000 = .eh_frame(0x2018) - .eh_frame_hdr(0x2004) - 4
 #    01000000 = 1 = the number of FDE pointers in the table.
@@ -43,10 +43,11 @@
 # CHECK-NEXT:   AddressAlignment:
 # CHECK-NEXT:   EntrySize:
 # CHECK-NEXT:   SectionData (
-# CHECK-NEXT:     0000: 0C000000 00000000 01520001 010102FF
-# CHECK-NEXT:     0010: 0C000000 14000000 34120000 00000000
-#                                           ^
-#                                           ---> ADDR(foo) + 0x234 = 0x1234
+# CHECK-NEXT:     0000: 14000000 00000000 01525300 01010102
+# CHECK-NEXT:     0010: FF000000 00000000 0C000000 1C000000
+# CHECK-NEXT:     0020: 34120000 00000000 00000000
+#                       ^
+#                       ---> ADDR(foo) + 0x234 = 0x1234
 
 .text
 .global foo
@@ -54,11 +55,12 @@ foo:
  nop
 
 .section .eh_frame,"a",@unwind
-  .long 12   # Size
+  .long 13   # Size
   .long 0x00 # ID
   .byte 0x01 # Version.
   
-  .byte 0x52 # Augmentation string: 'R','\0'
+  .byte 0x52 # Augmentation string: 'R','S','\0'
+  .byte 0x53
   .byte 0x00
   
   .byte 0x01
@@ -71,5 +73,5 @@ foo:
   .byte 0xFF
  
   .long 0x6  # Size
-  .long 0x14 # ID
+  .long 0x15 # ID
   .short foo + 0x234

lld/test/ELF/icf-eh-frame.s

@@ -29,6 +29,7 @@ _Z1bv:
 .section .text.Z1cv,"ax",@progbits
 _Z1cv:
   .cfi_startproc
+  .cfi_signal_frame
   ret
   .cfi_endproc