This patch doesn't change gdbserver behaviour, but after later changes are
made it avoids a null pointer dereference when HWCAP needs to be obtained
for a specific process while current_thread is nullptr.

Fixing linux_read_auxv, linux_get_hwcap and linux_get_hwcap2 to take a PID
parameter seems more correct than setting current_thread in one particular
code path.

Changes are propagated to allow passing the new parameter through the call
chain.

Approved-By: Simon Marchi <simon.marchi@efficios.com>