When printing an extended asm code fragment, placeholders %n
are replaced by register names.

Currently we ignore the fact that some assemblers use different
register names depending on the width of the data that resides
in the register.

For example, x86_64 uses %rax for a 64-bit quantity and %eax for
a 32-bit quantity, but CompCert always prints %rax in extended asm
statements.  This is problematic if we want to use 32-bit integer
instructions in extended asm, e.g.

             int x, y;
             asm("addl %1, %0", "=r"(x), "r"(y));
produces
             addl %rax, %rdx

which is syntactically incorrect.

Another example is ARM FP registers: D0 is a double-precision float,
but S0 is a single-precision float.

This commit partially solves this issue by taking into account the
Cminor type of the asm parameter when printing the corresponding register.
Continuing the previous example,

             int x, y;
             asm("addl %1, %0", "=r"(x), "r"(y));
now produces
             addl %eax, %edx

This is not perfect yet: we use Cminor types, because this is all we
have at hand, and not source C types, hence "char" and "short" parameters
are still printed like "int" parameters, which is not good for x86.
(I.e. we produce %eax where GCC might have produced %al or %ax.)
We'll leave this issue open.