Export mcontext fields on x86_64-unknown-linux-musl

[nmattia]

lucet uses some of the fields of mcontext that were added recently for x86_64-unknown-linux-gnu. It'd be great to port those to x86_64-unknown-linux-musl. In particular:

libc/src/unix/linux_like/linux/gnu/b64/x86_64/mod.rs

Lines 221 to 225 in 7b31ef3

	pub struct mcontext_t {
	pub gregs: [greg_t; 23],
	pub fpregs: *mut _libc_fpstate,
	__private: [u64; 8],
	}

and

libc/src/unix/linux_like/linux/gnu/b64/x86_64/mod.rs

Line 871 in 7b31ef3

pub const REG_RIP: ::c_int = 16;

which were ported in ae06c2b and ac7f0fa. Lucet only need mcontext.gregs and REG_RIP to be defined (see here).

These are defined in musl with GNU_SOURCE:

• https://github.com/ifduyue/musl/blob/b4b1e10364c8737a632be61582e05a8d3acf5690/arch/x86_64/bits/signal.h#L42
• https://github.com/ifduyue/musl/blob/b4b1e10364c8737a632be61582e05a8d3acf5690/arch/x86_64/bits/signal.h#L81

[gnzlbg]

PRs welcome :)

[nmattia]

@gnzlbg will do, just wanted to double-check first that the GNU_SOURCE requirement wasn't a dealbreaker.

[gnzlbg]

No it isn't, for linux targets (independently of whether they use gnu or musl), we always define it here: https://github.com/rust-lang/libc/blob/master/libc-test/build.rs#L2072