linkage: add generic GLOBAL() macro

Declaring a global symbol in assembly is tedious, error-prone, and
painful to read. While ENTRY() exists, this is supposed to be used for
function entry points, and this affects alignment in a potentially
undesireable manner.

Instead, let's add a generic GLOBAL() macro for this, as x86 added
locally in commit:

  95695547

 ("x86: asm linkage - introduce GLOBAL macro")

... thus allowing us to use this more freely in the kernel.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Torsten Duwe <duwe@suse.de>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>