Move processing_gcc to stabsread

Move processing_gcc to stabsread

processing_gcc is also only used by stabsread -- it is set by the
DWARF reader, but this turns out not to be needed.  So, this patch
moves processing_gcc and removes the assignment from the DWARF reader.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* stabsread.h (processing_gcc_compilation): Move from buildsym.h.
	* dwarf2read.c (dwarf2_start_symtab): Don't set
	processing_gcc_compilation.
	* buildsym.h (processing_gcc_compilation): Move to stabsread.h.