gh-131865: Properly apply exported CFLAGS for dtrace/systemtap builds (#131866)

When using --with-dtrace the resulting object file could be missing
specific CFLAGS exported by the build system due to the systemtap
script using specific defaults.

Exporting the CC and CFLAGS variables before the dtrace invocation
allows us to properly apply CFLAGS exported by the build system
even when cross-compiling.

The fix does not affect the dtrace invocation on Solaris/macOS.