libsframe: use uint8_t instead of unsigned char for abi_arch

libsframe: use uint8_t instead of unsigned char for abi_arch

Use uint8_t consistently for identifying ABI/arch in SFrame format.

bfd/
	* elf-sframe.c (_bfd_elf_merge_section_sframe):
libsframe/
	* sframe-dump.c (is_sframe_abi_arch_aarch64): Use uint8_t for
	local variable.
	* sframe.c (sframe_decoder_get_abi_arch): Update return type to
	uint8_t.
	(sframe_encoder_get_abi_arch): Likewise.
include/
	* sframe-api.h (sframe_decoder_get_abi_arch): Likewise.
	(sframe_encoder_get_abi_arch): Likewise.