MINOR: global: declare a read_mostly section

MINOR: global: declare a read_mostly section

Some variables are mostly read (mostly pointers) but they tend to be
merged with other ones in the same cache line, slowing their access down
in multi-thread setups. This patch declares an empty, aligned variable
in a section called "read_mostly". This will force a cache-line alignment
on this section so that any variable declared in it will be certain to
avoid false sharing with other ones. The section will be eliminated at
link time if not used.

A __read_mostly attribute was added to compiler.h to ease use of this
section.