add TARGET=gcp for Google Compute Engine BSP

[abarisani]

This PR adds a specific target for Google Cloud Compute in order to avoid re-use of the existing microvm target.

The PR requires usbarmory/tamago-go@992e3e2 and therefore should be merged only after tamago-go1.25.6.

ssh xxx.xx.155.105.bc.googleusercontent.com
...
> info
Runtime ......: go1.25.5 tamago/amd64 GOMAXPROCS=1
RAM ..........: 0x10000000-0x50000000 (1024 MiB)
Board ........: gcp
CPU ..........: AMD EPYC 7B13
Cores ........: 1
Frequency ....: 2.4499998 GHz
VirtIO Net0 ..: 42:01:0a:84:00:02

[andrejro]

lgtm