proc/native: optimize native.status through buffering (go-delve#1865)

Benchmark before:

BenchmarkConditionalBreakpoints-4              1        15649407130 ns/op

Benchmark after:

BenchmarkConditionalBreakpoints-4   	       1	14586710018 ns/op

Conditional breakpoint evaluation 1.56ms -> 1.45ms

Updates go-delve#1549

pkg/proc/native/proc_linux.go

@@ -1,6 +1,7 @@
 package native
 
 import (
+	"bufio"
 	"bytes"
 	"errors"
 	"fmt"
@@ -351,6 +352,7 @@ func status(pid int, comm string) rune {
 		return '\000'
 	}
 	defer f.Close()
+	rd := bufio.NewReader(f)
 
 	var (
 		p     int
@@ -361,7 +363,7 @@ func status(pid int, comm string) rune {
 	// The name of the task is the base name of the executable for this process limited to TASK_COMM_LEN characters
 	// Since both parenthesis and spaces can appear inside the name of the task and no escaping happens we need to read the name of the executable first
 	// See: include/linux/sched.c:315 and include/linux/sched.c:1510
-	fmt.Fscanf(f, "%d ("+comm+")  %c", &p, &state)
+	fmt.Fscanf(rd, "%d ("+comm+")  %c", &p, &state)
 	return state
 }