POSIX arch: Fix linker -T warning

Modify the linker script with the command INSERT.
It instructs the linker to augment the default linker script
SECTIONS with the ones provided with the one provided in
this script.
It also modified the meaning of the -T switch, so it no longer
replaces the default linker script

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>