[MC][ELF] Accept abbreviated form with sh_flags and sh_entsize
However, GNU assembler accepts and GCC generates an abbreviated form:
while the first .section contains the flags and entsize, subsequent
sections simply contain the name without repeating entsize or flags.
Do likewise for better compatibility.
Reviewed By: jhenderson, MaskRay
Differential Revision: https://reviews.llvm.org/D92052