De-pollute arm's sysarch.h.

Instead of including stdint.h for uintptr_t, include sys/_types.h and use
__types for everything that isn't a native C keyword type.

Remove the #include of cdefs.h. It appears after the include of armreg.h
which has a precondition of cdefs.h being included before it, so everyone
including sysarch.h is already including cdefs.h. (When armv5 support
goes away, there will be no need include armreg.h here either.)

Unfortunately, the unprefixed struct member names "addr" and "len" cannot
be changed, because 3rd-party software is relying on them (libcompiler_rt
is one known consumer).