gcc: provide option to enable mudflap support

The mudflap library is only useful if one uses the -fmudflap gcc
option to do more checks on pointers/arrays. This commit adds an
option to enable/disable mudflap support at the gcc level. By default,
it is disabled, which saves a little bit of build time compared to the
default of gcc which consists in enabling mudflap support.

Since mudflap is now disabled by default, and ensured to never be enabled
on platforms where it is not available, some gcc.mk code that was used to
disable mudflap in problematic configurations can be removed.

Whether -fmudflap is used when building is left to the user.

[Peter: tweak commit text, fix white space]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>