runtime.g is a large and growing struct, we only need a few fields.
Instead of using loadValue to load the full contents of g, cache its
memory and then only load the fields we care about.

Benchmark before:

BenchmarkConditionalBreakpoints-4              1        14586710018 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	12476166303 ns/op

Conditional breakpoint evaluation: 1.45ms -> 1.24ms

Updates #1549