debug/buildinfo: reuse buffer in searchMagic

debug/buildinfo: reuse buffer in searchMagic

Allocating a new buffer for each chunk in searchMagic is very
inefficient. Refactor reading to allow us to reuse the same buffer for
each iteration.

This reduces the runtime of `go version` on a 2.5GB non-Go binary from
~1s and ~25MB RSS to ~250ms and ~15MB RSS.

For #68592.

Change-Id: Idae5c2c9b3b8a7158d5cc7f2f008998be75fd7af
Reviewed-on: https://go-review.googlesource.com/c/go/+/601460
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>