Makefile: introduce PROG_NAME to contain the 'ct-ng' executable name

With the upcomming --program-{prefix,suffix,transform-name} options,
we'll have a need to centralise the actual executable name, that is
now no longer be a constant.

Rather than spread the prefix/suffix through-out the code, just
centralise the name setting in one place. Beside, transform-name
would not be possible without setting the name at a single location.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 diff -durN gcc-3.4.6.orig/gcc/config/mips/t-linux gcc-3.4.6/gcc/config/mips/t-linux

 --- gcc-3.4.6.orig/gcc/config/mips/t-linux	1970-01-01 01:00:00.000000000 +0100

 +++ gcc-3.4.6/gcc/config/mips/t-linux	2007-08-15 23:08:15.000000000 +0200

 @@ -0,0 +1,3 @@

 +# Compile crtbegin/end with xgot so it works for both

 +# normal and large GOTs.

 +CRTSTUFF_T_CFLAGS = -mxgot