build: factorise generation of the linker scripts

build: factorise generation of the linker scripts

In Arm and X86 makefile, generating the linker script is the same, so
we can simply have both call the same macro.

We need to add *.lds files into extra-y so that Rules.mk can find the
.*.cmd dependency file and load it.

Change made to the command line:
- Use cpp_flags macro which simply filter -Wa,% options from $(a_flags).
- Added -D__LINKER__ even it is only used by Arm's lds.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>