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