[MC][ELF] Accept abbreviated form with sh_flags and sh_entsize

D73999 / commit 75af9da7
added for LLVM 11 a check that sh_flags and sh_entsize (and sh_type)
changes are an error, in line with GNU assembler.

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.

See https://bugs.llvm.org/show_bug.cgi?id=48201

Reviewed By: jhenderson, MaskRay

Differential Revision: https://reviews.llvm.org/D92052