Fix integer overflow sanitizer in FastThread.cpp.

Fix the integer overflow sanitizer in the FastThread threadLoop()
function by using the builtin overflow-safe function.

 runtime error: unsigned integer overflow: 4294934528 + 65536 cannot be
 represented in type 'unsigned int'

Bug: 30969751
Test: Builds and boots.
Change-Id: I86e20932fb288deebe4fb778a689cf71eea6be79