[builtins] Use aliases for function redirects

Symbol aliases are supported by all platforms that compiler-rt builtins
target, and we can use these instead of function redirects to avoid the
extra indirection.

This is part of the cleanup proposed in "[RFC] compiler-rt builtins
cleanup and refactoring".

Differential Revision: https://reviews.llvm.org/D60931

llvm-svn: 359413