优化主线程获取栈顶/栈底耗时较大的问题 #42
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主线程第一次获取栈顶/栈底时会一个字节一个字节的读操作，这样会相当耗时，现在修改为逐行读取，粗略估算速度可提高10倍。
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主线程第一次获取栈顶/栈底时会一个字节一个字节的读操作，这样会相当耗时，现在修改为逐行读取，粗略估算速度可提高10倍。
主线程为什么不使用使用pthread_getattr_np？
（1）man pthread_getattr_np的ERRORS里有提到：
In addition, if thread refers to the main thread, then
pthread_getattr_np() can fail because of errors from various
underlying calls: fopen(3), if /proc/self/maps can't be opened;
and getrlimit(2), if the RLIMIT_STACK resource limit is not
supported.
（2）主线程使用pthread_getattr_np也是相当耗时，貌似也是需要解析/proc/self/maps，并且不止解析/proc/self/maps。
测试机型：
三星SM-G570F CPU架构armeabi-v7a Android版本7.0 测试OK
LG-H422 CPU架构armeabi-v7a Android版本5.0.1 测试OK
三星SM-N910F CPU架构aarmeabi-v7a Android版本4.4.2 测试OK
修改前后耗时对比：
