[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