lib/libgcc.c: Fix shift warnings

lib/libgcc.c: Fix shift warnings

```
  if (!(a & (0xffff << 16))) {
                    ^~
src/lib/libgcc.c:40:18: error: result of '255 << 24' requires 33 bits to represent, but 'int' only has 32 bits [-Werror=shift-overflow=]
  if (!(a & (0xff << 24))) {
                  ^~
src/lib/libgcc.c:45:17: error: result of '15 << 28' requires 33 bits to represent, but 'int' only has 32 bits [-Werror=shift-overflow=]
  if (!(a & (0xf << 28))) {
                 ^~
```

Change-Id: I7bdd75c20a76b303743d7e7e0d3a6503760284fd
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-on: https://review.coreboot.org/c/23361
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: HAOUAS Elyes <ehaouas@noos.fr>