builtin: fix compilation with tcc on OpenBSD using libgc

[lcheylus]

Add #flag to find libgc in /usr/local/lib/ on OpenBSD with tcc compiler.

Fix #26447

---

Tests OK for vlib/v/slow_tests/assembly/asm_test.amd64.v on OpenBSD with tcc compiler

$ ./v -stats test vlib/v/slow_tests/assembly/asm_test.amd64.v
---- Testing... ----
        V  source  code size:      31893 lines,     145791 tokens,     864019 bytes,   323 types,    14 modules,   151 files
generated  target  code size:       9700 lines,     360911 bytes
compilation took: 1411.998 ms, compilation speed: 22587 vlines/s, cgen threads: 3
running tests in: /home/fox/dev/vlang.git/vlib/v/slow_tests/assembly/asm_test.amd64.v
      OK    [1/6]     0.043 ms    13 asserts | main.test_inline_asm()
      OK    [2/6]     0.006 ms     1 assert  | main.test_rip_relative_label()
      OK    [3/6]     0.000 ms     1 assert  | main.test_rip_relative_label_u8()
      OK    [4/6]     0.001 ms     5 asserts | main.test_flag_output()
      OK    [5/6]     0.001 ms     4 asserts | main.test_asm_generic()
      OK    [6/6]     0.000 ms     2 asserts | main.test_lock_prefix()
     Summary for running V tests in "asm_test.amd64.v": 26 passed, 26 total. Elapsed time: 0 ms.

 OK    1472.203 ms vlib/v/slow_tests/assembly/asm_test.amd64.v
----
Summary for all V _test.v files: 1 passed, 1 total. Elapsed time: 1475 ms, on 1 job. Comptime: 0 ms. Runtime: 1472 ms.

All tests OK for vlib/v/slow_tests/assembly/ on OpenBSD/amd64 with tcc compiler

$ ./v test vlib/v/slow_tests/assembly/
---- Testing... ----
 SKIP  [ 1/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/asm_test.arm64.v
 SKIP  [ 2/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/asm_test.i386.v
 SKIP  [ 3/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/asm_test.loongarch64.v
 SKIP  [ 4/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/asm_test.ppc64le.v
 SKIP  [ 5/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/asm_test.rv64.v
 SKIP  [ 6/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/asm_test.s390x.v
 SKIP  [ 7/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/naked_attr_test.amd64.v
 SKIP  [ 8/11] C:     0.0 ms, R:     0.000 ms vlib/v/slow_tests/assembly/naked_attr_test.i386.v
 OK    [ 9/11] C:   821.1 ms, R:    12.094 ms vlib/v/slow_tests/assembly/asm_empty_statement_test.v
 OK    [10/11] C:  1565.8 ms, R:     6.462 ms vlib/v/slow_tests/assembly/asm_test.amd64.v
 OK    [11/11] C:   864.2 ms, R:  1451.974 ms vlib/v/slow_tests/assembly/stmt_separator_test.amd64.v
----
Summary for all V _test.v files: 3 passed, 8 skipped, 11 total. Elapsed time: 2367 ms, on 3 parallel jobs. Comptime: 3251 ms. Runtime: 1470 ms.

[JalonSolov]

V comes with thirdparty/libgc, which should be used by default.

The system libgc should only be used if you pass the extra compilation flag.

[lcheylus]

V comes with thirdparty/libgc, which should be used by default.

The system libgc should only be used if you pass the extra compilation flag.

No, not the case for FreeBSD and OpenBSD with tcc compiler, see code in vlib/builtin/builtin_d_gcboehm.c.v and my previous fix in commit c3b55a6

[spytheman]

(the CI failures are unrelated, sorry)

[spytheman]

I think that the asm test failure is only slightly related, but it is good to have that fixed too.

Thank you @lcheylus 🙇🏻 .