[crt] Remove __USER_LABEL_PREFIX__

The .init_array code is ELF specific. For ELF platforms,
`__USER_LABEL_PREFIX__` is defined as "". Make the simplification
so that downstream ELF targets can build this file even if
`__USER_LABEL_PREFIX__` is undefined.

Reviewed By: barannikov88

Differential Revision: https://reviews.llvm.org/D147093