cortex_m: linker: lld: Handle symtab/strtab/shstrtab to fix warnings

LLD produces warnings about orphaned sections if there are no matching
rules in the linker script:

ld.lld: warning: <internal>:(.symtab) is being placed in '.symtab'
ld.lld: warning: <internal>:(.shstrtab) is being placed in '.shstrtab'
ld.lld: warning: <internal>:(.strtab) is being placed in '.strtab'

Handle these sections when building with LLD to prevent the warnings.

Signed-off-by: Patryk Duda <patrykd@google.com>