www/mattermost-server: Remove i386 from supported platforms because modernc.org/libc does not build

vendor/modernc.org/libc/libc_freebsd.go:247:25: cannot use int64(off) (value of type int64) as int32 value in assignment
vendor/modernc.org/libc/libc_freebsd.go:269:50: cannot use int64(off) (value of type int64) as int32 value in assignment
vendor/modernc.org/libc/libc_freebsd.go:1622:17: invalid operation: Uint64(Uint64FromInt32(-1)) / uint64(255) * size_t(c) (mismatched types uint64 and size_t)
vendor/modernc.org/libc/libc_freebsd.go:1623:192: invalid operation: *(*uint64)(unsafe.Pointer(w)) ^ k (mismatched types uint64 and size_t)
vendor/modernc.org/libc/libc_freebsd.go:1642:42: cannot use &l1 (value of type *size_t) as *uint64 value in argument to AssignUint64
vendor/modernc.org/libc/libc_freebsd.go:1642:47: cannot use size_t((int64(X__strchrnul(tls, var1, '=')) - int64(var1)) / 1) (value of uint32 type size_t) as uint64 value in argument to AssignUint64
vendor/modernc.org/libc/libc_freebsd.go:1651:19: invalid operation: l1 + l2 + uint64(2) (mismatched types size_t and uint64)
vendor/modernc.org/libc/libc_freebsd.go:1657:48: invalid operation: l2 + uint64(1) (mismatched types size_t and uint64)
vendor/modernc.org/libc/libc_freebsd.go:1670:6: cannot use uint64(0) (constant 0 of type uint64) as size_t value in assignment
vendor/modernc.org/libc/libc_freebsd.go:1680:57: invalid operation: l + uint64(1) (mismatched types size_t and uint64)
vendor/modernc.org/libc/libc_freebsd.go:1680:57: too many errors