Add Prefer_MAP_32BIT_EXEC for Silvermonthjl/32bit/generic

According to Silvermont software optimization guide, for 64-bit
applications, branch prediction performance can be negatively impacted
when the target of a branch is more than 4GB away from the branch.  Set
the Prefer_MAP_32BIT_EXEC bit for Silvermont so that mmap will try to
map executable pages with MAP_32BIT first.  Also enable Silvermont
optimizations for Knights Landing.

Prefer_MAP_32BIT_EXEC reduces bits available for address space
layout randomization (ASLR), which is always disabled for SUID
programs and can only be enabled by setting environment variable,
LD_ENABLE_PREFER_MAP_32BIT_EXEC.

On Fedora 23, this patch speeds up GCC 5 testsuite by 3% on Silvermont.

	* sysdeps/unix/sysv/linux/wordsize-64/mmap.c: New file.
	* sysdeps/unix/sysv/linux/x86_64/64/mmap.c: Likewise.
	* sysdeps/x86/cpu-features.c (get_prefer_map_32bit_exec): New
	function.
	(init_cpu_features): Call get_prefer_map_32bit_exec for
	Silvermont.  Enable Silvermont optimizations for Knights Landing.
	* sysdeps/x86/cpu-features.h (bit_Prefer_MAP_32BIT_EXEC): New.
	(index_Prefer_MAP_32BIT_EXEC): Likewise.

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