libc/syscalls: Implement _write
libc/syscalls: Implement _write

All output goes to the UART. _write is used by printf(), so this allows
printf() to work correctly.

BRANCH=none
BUG=b:234181908, b:254530679
TEST=./test/run_device_tests.py --board dartmonkey -t libc_printf

Signed-off-by: Tom Hughes <fc5c12f30a5ed2baa0581f44ee14ad1eb7b20d83@chromium.org>
Change-Id: I60a5f283975c21dabac395f00b4738a6db083d29
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3997615
Reviewed-by: Bobby Casey <a3c107f0b4efaee07148ab0813ba992bba562a69@google.com>
Code-Coverage: Zoss <345cf107e82c70959d46f59d98bddc98ce458aff@prod.google.com>
Reviewed-by: Andrea Grandi <f017805e76e0e7a74072a3bac1b6e29e65eedeab@google.com>

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