zephyr: Import the compiler specific link flags
zephyr: Import the compiler specific link flags

I noticed earlier that the --coverage flag was missing from the link
command, and added it to ldd/linker_flags.cmake, but I realize this is
a better fix, as it will pick up all compiler specific link flags in
zephyr.

BUG=b:183007888
TEST=cmds in code_coverage.md
BRANCH=none

Change-Id: I957100dbe0e5c75e2e8124ce121f295778bd8f68
Signed-off-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2787353
Commit-Queue: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>
Tested-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>
Reviewed-by: Paul Fagerburg <d8c44b60afb21c60f56d931aa690ca1cf34910c8@chromium.org>
Reviewed-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec