Use linux_get_auxv to get AT_PHDR in the PPC stub

Use linux_get_auxv to get AT_PHDR in the PPC stub

This patch fixes a build error due to a call to ppc_get_auxv that was
left over after linux_get_hwcap and linux_get_hwcap2 were introduced
in:

974c89e0882ddb03e294eca76a9e3d3bef90eacf gdbserver: Add
linux_get_hwcap

Because the missing call fetched AT_PHDR and not AT_HWCAP,
linux_get_auxv is now visible.

This use also required ppc_get_auxv to return a status variable
indicating that the AT_PHDR entry was not found separately from the
actual value of of the auxv entry.  Therefore, the new linux_get_auxv
function is changed to return a status variable and write the entry
value to a pointer passed as an argument.

Note that linux_get_hwcap and linux_get_hwcap2 still use the return
value as both an indicator of that the entry wasn't found and as the
actual value of the entry.

gdb/gdbserver/ChangeLog:
2019-04-05  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* linux-low.c (linux_get_auxv): Remove static.  Return auxv entry
	value in argument pointer, return 1 if the entry is found and 0
	otherwise.  Move comment.
	(linux_get_hwcap, linux_get_hwcap2): Use modified linux_get_auxv.
	* linux-low.h (linux_get_auxv): Declare.
	* linux-ppc-low.c (is_elfv2_inferior): Use linux_get_auxv.