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