[phb]: update USE_GCC description

Update USE_GCC documentation after port's commit 768f18f327ac9646fb8ee83263233909be540966

Show that USE_GCC=any is deprecated and also the possibility to specify :build
to inject a BUILD only dependency.

PR: 254253
Reported by: gerald@
Reviewed by: 0mp@, gerald@
Approved by: 0mp@ (mentor), gerald@
Differential Revision: https://reviews.freebsd.org/D30209