Add Aarch64 SVE compatibility macros

Add Aarch64 SVE compatibility macros

This header provides compatibility support for SVE allow building
even when the underlying host system lacks support for SVE.
If the binary is then run on an SVE-enabled kernel then support
will automatically be available.

gdb/
	* nat/aarch64-sve-linux-sigcontext.h: New file.
	* nat/aarch64-sve-linux-ptrace.h (SVE_VQ_BYTES): Move to
	new files.
	(SVE_VQ_MIN): Likewise.
	(SVE_VQ_MAX): Likewise.
	(SVE_VL_MIN): Likewise.
	(SVE_VL_MAX): Likewise.
	(SVE_NUM_ZREGS): Likewise.
	(SVE_NUM_PREGS): Likewise.
	(sve_vl_valid): Likewise.
	(struct user_sve_header): Likewise.