builtin: fix compilation with tcc on OpenBSD using libgc

[lcheylus]

Fix #22156

Tests OK on OpenBSD/amd64 with:

• tcc installed from OpenBSD packages (pkg_add tcc)
• tcc built for OpenBSD/amd64 => see Add a prebuilt openbsd-amd64 binary tccbin#55
• clang 16.0.6
• gcc version 11.2.0

$ ./thirdparty/tcc/tcc.exe -v
tcc version 0.9.28rc 2024-09-14 HEAD@b8b6a5fd (x86_64 OpenBSD)
$ ./v -cc tcc run ~/tmp/hello_world.v
Hello world from V

$ /usr/local/bin/tcc  -v
tcc version 0.9.27 2023-07-05 mob@5b28165 (x86_64 OpenBSD)
$ ./v -cc /usr/local/bin/tcc run ~/tmp/hello_world.v
Hello world from V

$ ./v -cc clang run ~/tmp/hello_world.v
Hello world from V
$ ./v -cc egcc run ~/tmp/hello_world.v
Hello world from V

[spytheman]

Excellent work.