x32 Fast tracepoints: Customize jump pad address

x32 Fast tracepoints: Customize jump pad address

MAP_32BIT is ignored on x32, meaning the jump pad can end up somewhere
between 2GB and 4GB, too far away from the executable for 5-byte
relative jumps (JMP rel32).  So on x32, try explicitly placing the
jump pad near the middle of the available address space.

gdb/gdbserver/ChangeLog:
2016-08-19  Pedro Alves  <palves@redhat.com>

	* linux-amd64-ipa.c (alloc_jump_pad_buffer) [__ILP32__]: Try
	allocating around 0x80000000.