[ELF] - Support optional comma after output section command.

[grimar]

Summary

I found this when tried to link linux kernel with LLD:

https://github.com/torvalds/linux/blob/master/arch/x86/entry/vdso/vdso-layout.lds.S#L86

Output section command can have optional comma at the end:

.text		: { *(.text*) }			:text	=0x90909090,

It was documented about 3 years ago for binutils:
https://sourceware.org/ml/binutils/2014-04/msg00045.html

lld/trunk/ELF/LinkerScript.cpp

ScriptParser::readOutputSectionDescription(StringRef OutSec) {
   }
   Cmd->Phdrs = readOutputSectionPhdrs();
 
   if (consume("="))
     Cmd->Filler = readOutputSectionFiller(next());
   else if (peek().startswith("="))
     Cmd->Filler = readOutputSectionFiller(next().drop_front());
 
+  // Consume optional comma following output section command.
+  consume(",");
+
   return Cmd;
 }
 
 // Read "=<number>" where <number> is an octal/decimal/hexadecimal number.
 // https://sourceware.org/binutils/docs/ld/Output-Section-Fill.html
 //
 // ld.gold is not fully compatible with ld.bfd. ld.bfd handles
 // hexstrings as blobs of arbitrary sizes, while ld.gold handles them

lld/trunk/test/ELF/linkerscript/sections-padding.s

 # RUN:   | FileCheck --check-prefix=ERR2 %s
 # ERR2: invalid filler expression: 0x99XX
 
 ## Check case with space between '=' and expression:
 # RUN: echo "SECTIONS { .mysec : { *(.mysec*) } = 0x1122 }" > %t.script
 # RUN: ld.lld -o %t.out --script %t.script %t
 # RUN: llvm-objdump -s %t.out | FileCheck -check-prefix=YES %s
 
+## Check case with optional comma following output section command:
+# RUN: echo "SECTIONS { .mysec : { *(.mysec*) } =0x1122, .a : { *(.a*) } }" > %t.script
+# RUN: ld.lld -o %t.out --script %t.script %t
+# RUN: llvm-objdump -s %t.out | FileCheck -check-prefix=YES %s
+
 .section        .mysec.1,"a"
 .align  16
 .byte   0x66
 
 .section        .mysec.2,"a"
 .align  16
 .byte   0x66
 
 .globl _start
 _start:
  nop