Set orig_eax to -1 for Linux x86 platforms

Summary:
For Linux x86 based environments the orig_eax/orig_rax
register should be set to -1 to prevent the instruction pointer
to be decremented, which was the cause for the SIGILL exception.

Fix for Bug 23659

Reviewers: zturner, ashok.thirumurthi, mikesart, jingham, clayborg

Subscribers: clayborg, labath

Differential Revision: http://reviews.llvm.org/D11411

llvm-svn: 244875