internal/fuzz: temporarily work around test failures after dev.fuzz merge

- Skip test_fuzz_cache and test_fuzz_seed_corpus on 386.
- Skip worker benchmarks when race mode is enabled.
- Stub coverage function on platforms we haven't tested yet. It's
  causing package initialization to panic on aix/ppc64.

For #48504

Change-Id: I79318b52b11a33fca66476b5050445d07422ef36
Reviewed-on: https://go-review.googlesource.com/c/go/+/351117
Trust: Jay Conrod <jayconrod@google.com>
Trust: Katie Hockman <katie@golang.org>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

src/internal/fuzz/coverage.go

@@ -6,28 +6,9 @@ package fuzz
 
 import (
 	"fmt"
-	"internal/unsafeheader"
 	"math/bits"
-	"unsafe"
 )
 
-// coverage returns a []byte containing unique 8-bit counters for each edge of
-// the instrumented source code. This coverage data will only be generated if
-// `-d=libfuzzer` is set at build time. This can be used to understand the code
-// coverage of a test execution.
-func coverage() []byte {
-	addr := unsafe.Pointer(&_counters)
-	size := uintptr(unsafe.Pointer(&_ecounters)) - uintptr(addr)
-
-	var res []byte
-	*(*unsafeheader.Slice)(unsafe.Pointer(&res)) = unsafeheader.Slice{
-		Data: addr,
-		Len:  int(size),
-		Cap:  int(size),
-	}
-	return res
-}
-
 // ResetCovereage sets all of the counters for each edge of the instrumented
 // source code to 0.
 func ResetCoverage() {