Allow targets to define minimum alignment for global variables

This patch adds a new common code feature that allows platform code to
request minimum alignment of global symbols.  The background for this is
that on SystemZ, the most efficient way to load addresses of global symbol
is the LOAD ADDRESS RELATIVE LONG (LARL) instruction.  This instruction
provides PC-relative addressing, but only to *even* addresses.  For this
reason, existing compilers will guarantee that global symbols are always
aligned to at least 2.  [ Since symbols would otherwise already use a
default alignment based on their type, this will usually only affect global
objects of character type or character arrays. ]  GCC also allows creating
symbols without that extra alignment by using explicit "aligned" attributes
(which then need to be used on both definition and each use of the symbol).

To enable support for this with Clang, this patch adds a
TargetInfo::MinGlobalAlign variable that provides a global minimum for the
alignment of every global object (unless overridden via explicit alignment
attribute), and adds code to respect this setting.  Within this patch, no
platform actually sets the value to anything but the default 1, resulting
in no change in behaviour on any existing target.

This version of the patch incorporates feedback from reviews by
Eric Christopher and John McCall.  Thanks to all reviewers!

Patch by Richard Sandiford.