reflectlite: use less stack when reading struct fields

[davecheney]

Reflection on struct fields used too much stack. RawType.Field and Value.Field each built a complete rawStructField in an extra stack frame, and that frame was the deepest part of the call. On small goroutine stacks this is a problem: simavr has a 384-byte default stack, and its value went up and down in the past because tests overflowed it (32a7b8c, a94e03e, and the 512-byte setting for math.go in main_test.go). Code that calls Value.Field in a recursive walk, for example struct equality in the runtime (runtime.reflectValueEqual), reaches the limit quickly.

Stack use for a four-field struct, dev compared to this PR. Built at -opt=z with LLVM 22.1.8. The values come from the disassembly of complete programs: pushed registers, stack adjustment and return address of each frame in the deepest chain. runtime.alloc is not included, because it is the same before and after.

Below runtime.reflectValueEqual (Value.Field and its callees):

Target	dev	This PR
cortex-m-qemu	112 B	48 B
riscv-qemu	128 B	80 B
simavr	65 B	45 B

Below the caller of reflect.Type.Field, with Field called from several locations so that it is not inlined into one caller:

Target	dev	This PR
cortex-m-qemu	48 B	40 B
riscv-qemu	48 B	48 B
simavr	68 B	31 B

The gain is largest on AVR. On cortex-m and riscv, LLVM already inlined most of the old Field path, so the gain there is small.

[davecheney]

@jakebailey @dgryski gentle ping

[dgryski]

LGTM. Had a discussion with by bot about this and forgot to come back here with a response.

[davecheney]

@dgryski @jakebailey PTAL