runtime: eliminate rendundant loads gcPercent

The previous change was an automated change that made gcPercent a
type-safe atomic variable. However, that introduced a lot of redundant
formal atomic loads of the variable. Remove them by only loading once in
each case, and reusing the value.

Change-Id: I49647135f423574f94506d456d1cc390150fad02
Reviewed-on: https://go-review.googlesource.com/c/go/+/357795


Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>