Auto merge of #3027 - Phantomical:auxval, r=JohnTitor

linux: Add AT_SYSINFO_EHDR constant

When passed to [`getauxval(3)`][2] this is used to retrieve the base address of the vdso mapped into the current process. It is defined the architecture specific `auxvec.h` headers but [it is defined to the same value in all of them][1] so I've put it in `src/unix/linux_like/linux/mod.rs` instead of in the arch submodule.

[1]: https://sourcegraph.com/search?q=context:global+repo:%5Egithub%5C.com/torvalds/linux%24+%23define+AT_SYSINFO_EHDR&patternType=standard&sm=1
[2]: https://man7.org/linux/man-pages/man3/getauxval.3.html

libc-test/semver/linux.txt

@@ -181,6 +181,7 @@ AT_SECURE
 AT_SYMLINK_FOLLOW
 AT_SYMLINK_NOFOLLOW
 AT_UID
+AT_SYSINFO_EHDR
 B1000000
 B1152000
 B1500000

src/unix/linux_like/linux/mod.rs

@@ -1583,6 +1583,10 @@ pub const AT_HWCAP2: ::c_ulong = 26;
 
 pub const AT_EXECFN: ::c_ulong = 31;
 
+// defined in arch/<arch>/include/uapi/asm/auxvec.h but has the same value
+// wherever it is defined.
+pub const AT_SYSINFO_EHDR: ::c_ulong = 33;
+
 pub const GLOB_ERR: ::c_int = 1 << 0;
 pub const GLOB_MARK: ::c_int = 1 << 1;
 pub const GLOB_NOSORT: ::c_int = 1 << 2;