Currently both _MaxMem and _MaxArena32 represent the maximum arena
size on 32-bit hosts (except on MIPS32 where _MaxMem is confusingly
smaller than _MaxArena32).

Clean up sysAlloc so that it always uses _MaxMem, which is the maximum
arena size on both 32- and 64-bit architectures and is the arena size
we allocate auxiliary structures for. This lets us simplify and unify
some code paths and eliminate _MaxArena32.

Fixes #18651. mheap.sysAlloc currently assumes that if the arena is
small, we must be on a 32-bit machine and can therefore grow the arena
to _MaxArena32. This breaks down on darwin/arm64, where _MaxMem is
only 2 GB. As a result, on darwin/arm64, we only reserve spans and
bitmap space for a 2 GB heap, and if the application tries to allocate
beyond that, sysAlloc takes the 32-bit path, tries to grow the arena
beyond 2 GB, and panics when it tries to grow the spans array
allocation past its reserved size. This has probably been a problem
for several releases now, but was only noticed recently because
mapSpans didn't check the bounds on the span reservation until
recently. Most likely it corrupted the bitmap before. By using _MaxMem
consistently, we avoid thinking that we can grow the arena larger than
we have auxiliary structures for.

Change-Id: Ifef28cb746a3ead4b31c1d7348495c2242fef520
Reviewed-on: https://go-review.googlesource.com/35253Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Elias Naur <elias.naur@gmail.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>