runtime: Fix garbage collector issue with non 4kB system page size

PR go/60931

The go garbage collector tracks memory in terms of 4kB pages.
Most of the code checks getpagesize() at runtime and does the
right thing.

On a 64kB ppc64 box I see SEGVs in long running processes
which has been diagnosed as a bug in scavengelist.
scavengelist does a madvise(MADV_DONTNEED) without rounding
the arguments to the system page size.  A strace of one of the
failures shows the problem:

madvise(0xc211030000, 4096, MADV_DONTNEED) = 0

The kernel rounds the length up to 64kB and we mark 60kB of
valid data as no longer needed.

Round start up to a system page and end down before calling
madvise.
Merge branch 'master' of github.com:paranoiacblack/gcc