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

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.

Reviewers: uweigand, davidxl

Reviewed By: uweigand

Subscribers: llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D49161

llvm-svn: 336775

compiler-rt/lib/profile/GCDAProfiling.c

@@ -79,7 +79,7 @@ static FILE *output_file = NULL;
  * 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;