[clang-check-codegen][cfstring] Accept either @ or % for progbits to make ppc64be bots happy.

PPC64BE bots use % instead of @ for directives like progbits. Since CFString tests also
check asm output, they fail on the following:

  cfstring3.c:44:19: error: CHECK-ASM-ELF: expected string not found in input
  // CHECK-ASM-ELF: .section cfstring,"aw",@progbits
  <stdin>:30:2: note: possible intended match here
  .section cfstring,"aw",%progbits

Updating that check with a {{[@%]}}progbits regex to make those bots happy.
 

llvm-svn: 343044