Fix reading 32 bit gcov tag values on little-endian machines

[marco-c]

Summary

The write buffer contains signed chars, which means the shift operations caused values such as the arc tag value (0x01a10000) to be read incorrectly (0xffa10000).

This fixes a regression from https://reviews.llvm.org/D49132.

[uweigand]

Ah, I missed that, probably because on SystemZ char is unsigned by default ... Sorry about that.

The patch LGTM.

lib/profile/GCDAProfiling.c

  * Buffer that we write things into.
  */
 #define WRITE_BUFFER_SIZE (128 * 1024)
-static char *write_buffer = NULL;
+static unsigned char *write_buffer = NULL;
 static uint64_t cur_buffer_size = 0;
 static uint64_t cur_pos = 0;
 static uint64_t file_size = 0;