[MC][ELF] Fix accepting abbreviated form with sh_flags and sh_entsize

Followup to D92052 as I missed an issue as shown via GCC bug https://gcc.gnu.org/PR97827, namely: (e.g.) ".rodata." implies ELF::SHF_ALLOC.

Crossref:

- 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.

-  D92052 / commit 1deff400
   permitted the abbreviated form which many assemblers accept and
   GCC generates: while the first .section contains the flags and entsize,
   subsequent sections simply contain the name without repeating entsize or
   flags.

However, the latter patch missed in the check that some flags are automatically set, e.g. '.rodata." implies ELF::SHF_ALLOC.

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

Reviewed By: jhenderson

Differential Revision: https://reviews.llvm.org/D94072