[libunwind] compile error sc_rbs_base

[Cohn, Robert]

I could not compile 0.2 on my redhat 7.2 machine because sc_rbs_base was not
a member of sigcontext. sigcontext comes from
/usr/include/bits/sigcontext.h. I found another copy in asm/sigcontext.h
that has the needed member. I tried replacing bits/sigcontext.h with the one
from asm, but ran into other problems. In the end, I copied the missing
fields from asm/sigcontext.h to bits/sigcontext.h. Is there a better way to
fix this problem?

Robert