parisc: avoid undefined shift in cnv_float.h

The attached change fixes a float conversion problem found running the
GCC testsuite with GCC configured with --with-arch=2.0.

The actual problem occurs for an exponent value of 63. This is the
maximum exponent value that can be passed. This causes a left shift by
32 in the else hunk of the macro. This causes undefined behavior and the
wrong value is returned for dresultB. The fix is the check "exponent <=
62". If the exponent is 63, dresultB is set to 0. The patch also
optimizes the operation a bit by copying "Sall(sgl_value) <<
SGL_EXP_LENGTH" to val, so that sgl_value is not modified.

Signed-off-by: John David Anglin <dave.anglin@bell.net>
Signed-off-by: Helge Deller <deller@gmx.de>