Fix long double support for ARM.

Fixes #15056.

Change-Id: I48c00b955346971aa88b731ccad1953a4044983d
Reviewed-on: https://review.haiku-os.org/c/1442
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>

Synchronize sysdep.h with current glibc

So that I can more easily import sparc headers from it.

NO_UNDERSCORES is gone and is now always the default.
Some previously arch-specific defines are now valid for all platforms.

Change-Id: If9876b241719559bdcb5cd9d8b1dc97e5e3d96b3
Reviewed-on: https://review.haiku-os.org/c/1141
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

libroot: ARM does not have regparm.

Also fix the test for USE_REGPARMS in the same file. Gets the Clang
build somewhat further.

ARM/libroot: copy some headers from x86

These are pretty generic 32bit target files, so just copy them over.
Once we get userland properly starting, we can review these to see
if they need any changes.

ARM/libroot: copy some headers from x86

These are pretty generic 32bit target files, so just copy them over.
Once we get userland properly starting, we can review these to see
if they need any changes.