gdb/linux: add helpers to read AT_HWCAP3 and AT_HWCAP4

Add linux_get_hwcap3 and linux_get_hwcap4 helpers to GDB and gdbserver,
mirroring the existing linux_get_hwcap and linux_get_hwcap2 interfaces.

The new helpers retrieve the AT_HWCAP3 and AT_HWCAP4 auxiliary vector
entry either from explicitly supplied auxv data or from the current
inferior.

This prepares for future features that depend on HWCAP3 and HWCAP4
capability bits.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Approved-By: Simon Marchi <simon.marchi@efficios.com>