runtime: reserve fewer memory for aligned reservation on sbrk systems
Sometimes the runtime needs to reserve some memory with a large
alignment, which the OS usually won't directly satisfy. So, it
asks size+align bytes instead, and frees the unaligned portions.
On sbrk systems, this doesn't work that well, as freeing the tail
portion doesn't really free the memory to the OS. Instead, we
could simply round the current break up, then reserve the given
size, without wasting the tail portion.
Also, don't create heap arena hints on sbrk systems. We can only
grow the break sequentially, and reserving specific addresses
would not succeed anyway.
For #69018.
Change-Id: Iadc2c54d62b00ad7befa5bbf71146523483a8c47
Reviewed-on: https://go-review.googlesource.com/c/go/+/621715 Reviewed-by: Michael Knyszek <mknyszek@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Michael Pratt <mpratt@google.com>