fix riscv64 signal.h namespace violations and ucontext API mismatches

fix riscv64 signal.h namespace violations and ucontext API mismatches

the top-level mcontext_t member names were namespace-violating in
standards profiles before, and nested-level member names (some of them
single-letter) were egregiously bad namespace impositions even in
non-strict profiles. moreover, they mismatched those used in the
public API first defined in glibc, breaking any code making use of
them.

unlike most archs, the public API used in glibc for riscv mcontext_t
members was designed to be namespace-safe, so we can and should expose
the members regardless of feature test macros. only the typedefs for
greg_t, gregset_t, and fpregset_t need to be protected behind FTMs.

the struct tags for mcontext_t and ucontext_t are also changed. for
mcontext_t this is necessary to make the common definition across
profiles namespace-safe. for ucontext_t, it's just a matter of
matching the tag from the glibc-defined API.

these changes are split off and expanded from a patch by Khem Raj.