cmd/compile, cmd/internal: fine-grained fiddling with loop alignment

This appears to be useful only on amd64, and was specifically
benchmarked on Apple Silicon and did not produce any benefit there.
This CL adds the assembly instruction `PCALIGNMAX align,amount`
which aligns to `align` if that can be achieved with `amount`
or fewer bytes of padding. (0 means never, but will align the
enclosing function.)

Specifically, if low-order-address-bits + amount are
greater than or equal to align; thus, `PCALIGNMAX 64,63` is
the same as `PCALIGN 64` and `PCALIGNMAX 64,0` will never
emit any alignment, but will still cause the function itself
to be aligned to (at least) 64 bytes.

Change-Id: Id51a056f1672f8095e8f755e01f72836c9686aa3
Reviewed-on: https://go-review.googlesource.com/c/go/+/577935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/base/debug.go

@@ -16,6 +16,7 @@ var Debug DebugFlags
 // The -d option takes a comma-separated list of settings.
 // Each setting is name=value; for ints, name is short for name=1.
 type DebugFlags struct {
+	AlignHot              int    `help:"enable hot block alignment (currently requires -pgo)" concurrent:"ok"`
 	Append                int    `help:"print information about append compilation"`
 	Checkptr              int    `help:"instrument unsafe pointer conversions\n0: instrumentation disabled\n1: conversions involving unsafe.Pointer are instrumented\n2: conversions to unsafe.Pointer force heap allocation" concurrent:"ok"`
 	Closure               int    `help:"print information about closure compilation"`