gdbserver: Add linux_get_hwcap

gdbserver: Add linux_get_hwcap

In gdbserver, Tidy up calls to read HWCAP (and HWCAP2) by adding common
functions, removing the Arm, AArch64, PPC and S390 specific versions.

No functionality differences.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (aarch64_get_hwcap): Remove function.
	(aarch64_arch_setup): Call linux_get_hwcap.
	* linux-arm-low.c (arm_get_hwcap): Remove function.
	(arm_read_description): Call linux_get_hwcap.
	* linux-low.c (linux_get_auxv): New function.
	(linux_get_hwcap): Likewise.
	(linux_get_hwcap2): Likewise.
	* linux-low.h (linux_get_hwcap): New declaration.
	(linux_get_hwcap2): Likewise.
	* linux-ppc-low.c (ppc_get_auxv): Remove function.
	(ppc_arch_setup): Call linux_get_hwcap.
	* linux-s390-low.c (s390_get_hwcap): Remove function.
	(s390_arch_setup): Call linux_get_hwcap.