valgrind client requests for undefined values

with this change, when you assign undefined, zig emits a few
assembly instructions to tell valgrind that the memory is undefined

it's on by default for debug builds, and disabled otherwise. only
support for linux, darwin, solaris, mingw on x86_64 is currently
implemented.

--disable-valgrind turns it off even in debug mode.
--enable-valgrind turns it on even in release modes.

It's always disabled for compiler_rt.a and builtin.a.

Adds `@import("builtin").valgrind_support` which lets code know
at comptime whether valgrind client requests are enabled.

See #1989

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