[InstrProf][compiler-rt] Fix counter section alignment issue

I recently discovered that `.profraw` headers are expected to be 8 byte
aligned.
https://github.com/llvm/llvm-project/blob/643ba926c1f618401c86dc37e659df795db2e1a0/llvm/lib/ProfileData/InstrProfReader.cpp#L503-L506

When function entry coverage mode is used, function counters are single
bytes, so it is likely that the size of the counters section is not 8
byte aligned. We can add padding after the counters section to guarantee
this.

Reviewed By: kyulee, gulfem

Differential Revision: https://reviews.llvm.org/D152479