Merge GCC producer parsers. Allow digits in identifiers.

Merge GCC producer parsers. Allow digits in identifiers.

Both dwarf2read.c (checkproducer) and utils.c (producer_is_gcc_ge_4)
implemented a GCC producer parser that tried to extract the major and minor
version of GCC. Merge them into one GCC producer parser used by both. Also
allow digits in the identifier after "GNU " such as used by GCC5 like:
"GNU C11 5.0.0 20150123 (experimental) -mtune=generic -march=x86-64 -gdwarf-5"

gdb/ChangeLog:

	* dwarf2read.c (checkproducer): Call producer_is_gcc.
	* utils.c (producer_is_gcc_ge_4): Likewise.
	(producer_is_gcc): New function.
	* utils.h (producer_is_gcc): New declaration.