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

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

Differential revision: https://reviews.llvm.org/D28803

llvm-svn: 292225
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 887ca12..d6d4c74 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -1471,6 +1471,9 @@
   else if (peek().startswith("="))
     Cmd->Filler = readOutputSectionFiller(next().drop_front());
 
+  // Consume optional comma following output section command.
+  consume(",");
+
   return Cmd;
 }