[AArch64] Make assert messages uniform and general [NFC]

Summary: Make assert messages related to Darwin, ELF and COFF uniform.

Reviewers: rnk, ruiu, compnerd, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, aemerson, rengolin, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D34730

llvm-svn: 306589