Unified sysroot: kill arch-specific include dirs.

<machine/asm.h> was internal use only.

<machine/fenv.h> is quite large, but can live in <bits/...>.

<machine/regdef.h> is trivially replaced by saying $x instead of x in
our assembler.

<machine/setjmp.h> is trivially inlined into <setjmp.h>.

<sgidefs.h> is unused.

Bug: N/A
Test: builds
Change-Id: Id05dbab43a2f9537486efb8f27a5ef167b055815