Detect SVE when reading aarch64 core files

Detect SVE when reading aarch64 core files

The SVE section in a core file contains a header followed by the registers.
Add defines to easily access the header fields within a buffer.

gdb/
	* aarch64-linux-tdep.c (SVE_HEADER_SIZE_LENGTH): Add define.
	(SVE_HEADER_MAX_SIZE_LENGTH): Likewise.
	(SVE_HEADER_VL_LENGTH): Likewise.
	(SVE_HEADER_MAX_VL_LENGTH): Likewise.
	(SVE_HEADER_FLAGS_LENGTH): Likewise.
	(SVE_HEADER_RESERVED_LENGTH): Likewise.
	(SVE_HEADER_SIZE_OFFSET): Likewise.
	(SVE_HEADER_MAX_SIZE_OFFSET): Likewise.
	(SVE_HEADER_VL_OFFSET): Likewise.
	(SVE_HEADER_MAX_VL_OFFSET): Likewise.
	(SVE_HEADER_FLAGS_OFFSET): Likewise.
	(SVE_HEADER_RESERVED_OFFSET): Likewise.
	(SVE_HEADER_SIZE): Likewise.
	(aarch64_linux_core_read_vq): Add function.
	(aarch64_linux_core_read_description): Check for SVE section.