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
diff --git a/libc/include/fenv.h b/libc/include/fenv.h
index ae35ae3..4276e87 100644
--- a/libc/include/fenv.h
+++ b/libc/include/fenv.h
@@ -31,7 +31,18 @@
 #define _FENV_H_
 
 #include <sys/cdefs.h>
-#include <machine/fenv.h>
+
+#if defined(__aarch64__)
+#include <bits/fenv_arm64.h>
+#elif defined(__arm__)
+#include <bits/fenv_arm.h>
+#elif defined(__i386__)
+#include <bits/fenv_x86.h>
+#elif defined(__mips__)
+#include <bits/fenv_mips.h>
+#elif defined(__x86_64__)
+#include <bits/fenv_x86_64.h>
+#endif
 
 __BEGIN_DECLS