[HWASan] Use less Printf() calls in register dump.

Summary:
Explicitly print 4 registers/line in each iteration during register
dump. Reduces logcat spam as we get a single logcat message per call to
Printf(), even if the output isn't newline-terminated. This brings the
output format in logcat closer to that of the normal textual dump.

Reviewers: eugenis, pcc

Reviewed By: pcc

Subscribers: kubamracek, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D59320

llvm-svn: 356166