Teach GDB that wchar_t is a built-in type in C++ mode

Teach GDB that wchar_t is a built-in type in C++ mode

GDB is currently not aware that wchar_t is a built-in type in C++
mode.  This is usually not a problem because the debug info describes
the type, so when you have a program loaded, you don't notice this.
However, if you try expressions involving wchar_t before a program is
loaded, gdb errors out:

 (gdb) p (wchar_t)-1
 No symbol table is loaded.  Use the "file" command.
 (gdb) p L"hello"
 No type named wchar_t.
 (gdb) ptype L"hello"
 No type named wchar_t.

This commit teaches gdb about the type.  After:

 (gdb) p (wchar_t)-1
 $1 = -1 L'\xffffffff'
 (gdb) p L"hello"
 $2 = L"hello"
 (gdb) ptype L"hello"
 type = wchar_t [6]

Unlike char16_t/char32_t, unfortunately, the underlying type of
wchar_t is implementation dependent, both size and signness.  So this
requires adding a couple new gdbarch hooks.

I grepped the GCC code base for WCHAR_TYPE and WCHAR_TYPE_SIZE, and it
seems to me that the majority of the ABIs have a 4-byte signed
wchar_t, so that's what I made the default for GDB too.  And then I
looked for which ports have a 16-bit and/or unsigned wchar_t, and made
GDB follow suit.

gdb/ChangeLog:
2017-04-12  Pedro Alves  <palves@redhat.com>

	PR gdb/21323
	* c-lang.c (cplus_primitive_types) <cplus_primitive_type_wchar_t>:
	New enum value.
	(cplus_language_arch_info): Register cplus_primitive_type_wchar_t.
	* gdbtypes.h (struct builtin_type) <builtin_wchar>: New field.
	* gdbtypes.c (gdbtypes_post_init): Create the "wchar_t" type.
	* gdbarch.sh (wchar_bit, wchar_signed): New per-arch values.
	* gdbarch.h, gdbarch.c: Regenerate.
	* aarch64-tdep.c (aarch64_gdbarch_init): Override
	gdbarch_wchar_bit and gdbarch_wchar_signed.
	* alpha-tdep.c (alpha_gdbarch_init): Likewise.
	* arm-tdep.c (arm_gdbarch_init): Likewise.
	* avr-tdep.c (avr_gdbarch_init): Likewise.
	* h8300-tdep.c (h8300_gdbarch_init): Likewise.
	* i386-nto-tdep.c (i386nto_init_abi): Likewise.
	* i386-tdep.c (i386_go32_init_abi): Likewise.
	* m32r-tdep.c (m32r_gdbarch_init): Likewise.
	* moxie-tdep.c (moxie_gdbarch_init): Likewise.
	* nds32-tdep.c (nds32_gdbarch_init): Likewise.
	* rs6000-aix-tdep.c (rs6000_aix_init_osabi): Likewise.
	* sh-tdep.c (sh_gdbarch_init): Likewise.
	* sparc-tdep.c (sparc32_gdbarch_init): Likewise.
	* sparc64-tdep.c (sparc64_init_abi): Likewise.
	* windows-tdep.c (windows_init_abi): Likewise.
	* xstormy16-tdep.c (xstormy16_gdbarch_init): Likewise.

gdb/testsuite/ChangeLog:
2017-04-12  Pedro Alves  <palves@redhat.com>

	PR gdb/21323
	* gdb.cp/wide_char_types.c: Include <wchar.h>.
	(wchar): New global.
	* gdb.cp/wide_char_types.exp (wide_char_types_program)
	(do_test_wide_char, wide_char_types_no_program, top level): Add
	wchar_t testing.