builder, cgen: fix macOS tcc header lookup

[guweigang]

Summary

• declare posix_memalign in generated C headers so macOS aligned_alloc fallback compiles cleanly under tcc -cstrict
• make parallel-cc locate bundled tcc headers/runtime from both lib/tcc and lib layouts
• add an absolute bundled tcc include path for parallel C compile shards that run from tmp dirs

Root Cause

macOS fallback code for aligned_alloc calls posix_memalign, but the manual libc declarations did not include a posix_memalign prototype. In cstrict mode, tcc treats that implicit declaration as an error and V falls back to the system compiler.

parallel-cc also assumed the older bundled tcc lib/tcc layout. The current macOS arm64 tcc bundle has headers under thirdparty/tcc/lib/include, and tcc's -B path alone is cwd-sensitive there, so compile shards created in /tmp could fail to include stdbool.h through Apple SDK headers.

Validation

• ./v -g -keepc -o ./vnew cmd/v
• VMODULES=/tmp/vmodules_codex ./vnew -cc tcc -cstrict -show-c-output run examples/hello_world.v
• VMODULES=/tmp/vmodules_codex ./vnew -silent vlib/v/builder/parallel_cc_failure_test.v
• VMODULES=/tmp/vmodules_codex ./vnew -silent vlib/v/compiler_errors_test.v
• VMODULES=/tmp/vmodules_codex ./vnew -silent vlib/v/gen/c/coutput_test.v

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