Makefile: Optimize dirname invocation

Invoking `dirname` for every single source file is slow. This change
switches to the `-exec dirname {} +` variant that passes multiple args
to `dirname`. With this change I no longer see `dirname` in `top` when
watching a build.

BUG=b:392102378
BRANCH=none
TEST=emerge-brya coreboot

Change-Id: I786b91df9516c70b34ff534c4144e228104bea2d
Signed-off-by: Raul E Rangel <[email protected]>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/6198688
Reviewed-by: Julius Werner <[email protected]>
Commit-Queue: Julius Werner <[email protected]>
1 file changed