runtime: on plan9 don't return substitute address for sysReserve

Plan 9 doesn't have a way to reserve virtual memory, so the
implementation of sysReserve allocates memory space (which won't
be backed with real pages until the virtual pages are referenced).
If the space is then freed with sysFree, it's not returned to
the OS (because Plan 9 doesn't allow shrinking a shared address
space), but it must be cleared to zeroes in case it's reallocated
subsequently.

This interacts badly with the way mallocinit on 64-bit machines
sets up the heap, calling sysReserve repeatedly for a very large
(64MB?) arena with a non-nil address hint, and then freeing the space
again because it doesn't have the expected alignment.  The
repeated clearing of multiple megabytes adds significant startup
time to every go program.

We correct this by restricting sysReserve to allocate memory only
when the caller doesn't provide an address hint.  If a hint is
provided, sysReserve will now return nil instead of allocating memory
at a different address.

Fixes #27744

Change-Id: Iae5a950adefe4274c4bc64dd9c740d19afe4ed1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/207917
Run-TryBot: David du Colombier <0intro@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/runtime/mem_plan9.go

@@ -193,7 +193,7 @@ func sysReserve(v unsafe.Pointer, n uintptr) unsafe.Pointer {
 		// so try to extend the address space.
 		p = sbrk(n)
 	}
-	if p == nil {
+	if p == nil && v == nil {
 		p = memAlloc(n)
 		memCheck()
 	}