toolchain: gcc: Simplify `GEN_ABSOLUTE_SYM` and `GEN_ABSOLUTE_SYM_KCONFIG`

The assembler code in these macros can be expressed using
target-independent language constructs. This change unifies the variants
from using a target-specific dialects to a target-agnostic solution.

This fixes building Zephyr for target AArch32, by using target-agnostic
`STT_OBJECT`.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>