Change gdbarch_wchar_bit for AArch64 and ARM

Change gdbarch_wchar_bit for AArch64 and ARM

The size of wchar_t on AArch64 and ARM is 4-byte, so we can use the
default value (4*TARGET_CHAR_BIT).

This patch fixes some fails in gdb.cp/wide_char_types.exp on
aarch64-linux.

gdb:

2017-04-25  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (aarch64_gdbarch_init): Don't call
	set_gdbarch_wchar_bit.
	* arm-tdep.c (arm_gdbarch_init): Likewise.