parisc: Makefile: explain that 64BIT requires both 32-bit and 64-bit compilers

For building a 64-bit kernel, both 32-bit and 64-bit VDSO binaries
are built, so both 32-bit and 64-bit compilers (and tools) should be
in the PATH environment variable.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "James E.J. Bottomley" <James.Bottomley@HansenPartnership.com>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>
Cc: stable@vger.kernel.org # v5.3+