compile: Add 'set compile-gcc'

compile: Add 'set compile-gcc'

As discussed in
	How to use compile & execute function in GDB
	https://sourceware.org/ml/gdb/2015-04/msg00026.html

GDB currently searches for compilers on /usr/bin/ARCH-OS-gcc and
chooses a match from there.  However, it is not currently possible for
the user to override which compiler to use.  This is what this patch
implements.

It is also a sync between GCC's and GDB's interfaces.

gdb/ChangeLog
2017-08-23  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* NEWS (Changes since GDB 7.9): Add set compile-gcc and show
	compile-gcc.
	* compile/compile.c (compile_gcc, show_compile_gcc): New.
	(compile_to_object): Implement compile_gcc.
	(_initialize_compile): Install "set compile-gcc".  Initialize
	compile_gcc.

gdb/doc/ChangeLog
2017-08-23  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gdb.texinfo (Compiling and Injecting Code): Add to subsection
	"Compiler search for the compile command" descriptions of set
	compile-gcc and show compile-gcc.

include/ChangeLog
2017-08-23  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* gcc-interface.h (enum gcc_base_api_version): Update comment for
	GCC_FE_VERSION_1.
	(struct gcc_base_vtable): Rename set_arguments to set_arguments_v0.
	Add set_arguments, set_triplet_regexp and set_driver_filename.