reduce impact of REG_* namespace pollution in x86[_64] signal.h

reduce impact of REG_* namespace pollution in x86[_64] signal.h

when _GNU_SOURCE is defined, which is always the case when compiling
c++ with gcc, these macros for the the indices in gregset_t are
exposed and likely to clash with applications. by using enum constants
rather than macros defined with integer literals, we can make the
clash slightly less likely to break software. the macros are still
defined in case anything checks for them with #ifdef, but they're
defined to expand to themselves so that non-file-scope (e.g.
namespaced) identifiers by the same names still work.

for the sake of avoiding mistakes, the changes were generated with sed
via the command:

sed -i -e 's/#define  *\(REG_[A-Z_0-9]\{1,\}\)  *\([0-9]\{1,\}\)'\
'/enum { \1 = \2 };\n#define \1 \1/' \
arch/i386/bits/signal.h arch/x86_64/bits/signal.h arch/x32/bits/signal.h