Remove arch_get_caller

Use the gcc builtin instead, which generates more efficient code (it
saves a function call) and means less platform specific code to write
for us.

Change-Id: I1d55b5703027b2ea4ecde2438ea306bd4850eb32
Reviewed-on: https://review.haiku-os.org/c/haiku/+/1859
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

sparc: get libroot to build

Add various stubs to fix undefined references. No implementation for
anything yet.

Change-Id: I2d398bc2369d099e3a35f0713058d6a5edc6801d
Reviewed-on: https://review.haiku-os.org/c/1138
Reviewed-by: waddlesplash <waddlesplash@gmail.com>