[compiler-rt] Simplify ALIAS() attribute macro

Most uses of ALIAS() are in conjunction with WRAPPER_NAME().

Simplify the code and just make ALIAS() turn its argument into a string
(similar to Linux kernel's __alias macro). This in turn allows removing
WRAPPER_NAME().

NFC.

Reviewed By: dvyukov

Differential Revision: https://reviews.llvm.org/D151216