Cleanup `.globl` usage

In some places we use `ENTRY()` to mark an assembly function, whereas in
others we just use `.globl`. Where we use `.globl` for functions or
data, we don't keep this close to the actual definition.

Further, `ENTRY()` is a keyword in linker script with a different
meaning, and so it would be nicer if we didn't use the same term in the
assembly files.

This patch adds `ASM_FUNC()` and `ASM_DATA()` markers, and uses them
consistently throughout the codebase.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>