toolchain: generate check-headers program under $(BUILD_DIR)

Some installations mount /tmp with the 'noexec' option, which prevents
running the program generated there to check the kernel headers.

Avoid the problem by generating the program under $(BUILD_DIR), passed
as the first argument to check-kernel-headers.sh.

We could globally export a TMPDIR environment variable with some path
under $(BUILD_DIR) but such solution would be too intrusive, depriving
the user from the freedom to set TMPDIR at his will (or needs).

Fixes: https://bugs.busybox.net/show_bug.cgi?id=12241



Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>