IPA: Add alloc_jump_pad_buffer target hook.

IPA: Add alloc_jump_pad_buffer target hook.

Targets may have various requirements on the required location of the jump
pad area.  Currently IPA allocates it at the lowest possible address,
so that it is reachable by branches from the executable.  However, this
fails on powerpc, which has executable link address (0x10000000) much
larger than branch reach (+/- 32MiB).

This makes jump pad buffer allocation a target hook instead.  The current
implementations are as follows:

- i386 and s390: Branches can reach anywhere, so just mmap it.  This
  avoids the linear search dance.
- x86_64: Branches have +/-2GiB of reach, and executable is loaded low,
  so just call mmap with MAP_32BIT.  Likewise avoids the linear search.
- aarch64: Branches have +-128MiB of reach, executable loaded at 4MiB.
  Do a linear search from 4MiB-size downwards to page_size.
- s390x: Branches have +-4GiB of reach, executable loaded at 2GiB.
  Do like on aarch64.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-ipa.c: Add <sys/mman.h> and <sys/auxv.h> includes.
	(alloc_jump_pad_buffer): New function.
	* linux-amd64-ipa.c: Add <sys/mman.h> include.
	(alloc_jump_pad_buffer): New function.
	* linux-i386-ipa.c (alloc_jump_pad_buffer): New function.
	* linux-s390-ipa.c: Add <sys/mman.h> and <sys/auxv.h> includes.
	(alloc_jump_pad_buffer): New function.
	* tracepoint.c (getauxval) [!HAVE_GETAUXVAL]: New function.
	(initialize_tracepoint): Delegate to alloc_jump_pad_buffer.
	* tracepoint.h (alloc_jump_pad_buffer): New prototype.
	(getauxval) [!HAVE_GETAUXVAL]: New prototype.