ARM: make gd a function for clang

ARM: make gd a function for clang

"clang does not support global register variables; this is
unlikely to be implemented soon because it requires additional
LLVM backend support" [1]

Workaround it by obtaining the value of gd/r9 by an inline
asm routine. Note there is no set routine added for ARM at the
moment, since most if not all updates of gd from c are actually
not needed for ARM.

[1] http://clang.llvm.org/docs/UsersManual.html

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>