[SPARC] Assembler does not recognize `illtrap`

[alexrp]

In glibc's start code, unimp is used on 32-bit; LLVM already recognizes that.

But for 64-bit, illtrap is used instead, which LLVM currently does not recognize. Assembling glibc's start code currently fails as a result.

[brad0]

@koachan