Add PR_SET_MEMORY_MERGE and PR_GET_MEMORY_MERGE for linux

Add `PR_SET_MEMORY_MERGE` and `PR_GET_MEMORY_MERGE` prctl constants for the linux target. These control [per-process opt-in to KSM](https://lwn.net/Articles/953141/) and have been available since kernel 6.4. They were already defined for the android target but missing from linux

(cherry picked from commit 545663c)

Author: fspv

libc-test/build.rs

@@ -4422,6 +4422,8 @@ fn test_linux(target: &str) {
                 | "PR_MDWE_NO_INHERIT"
                 | "PR_MDWE_REFUSE_EXEC_GAIN"
                 | "PR_SET_MDWE"
+                | "PR_GET_MEMORY_MERGE"
+                | "PR_SET_MEMORY_MERGE"
                 | "IPPROTO_ETHERNET"
                 | "IPPROTO_MPTCP"
                 | "SI_DETHREAD"

libc-test/semver/linux.txt

@@ -2404,6 +2404,7 @@ PR_GET_FPEXC
 PR_GET_FP_MODE
 PR_GET_KEEPCAPS
 PR_GET_MDWE
+PR_GET_MEMORY_MERGE
 PR_GET_NAME
 PR_GET_NO_NEW_PRIVS
 PR_GET_PDEATHSIG
@@ -2443,6 +2444,7 @@ PR_SET_FPEXC
 PR_SET_FP_MODE
 PR_SET_KEEPCAPS
 PR_SET_MDWE
+PR_SET_MEMORY_MERGE
 PR_SET_MM
 PR_SET_MM_ARG_END
 PR_SET_MM_ARG_START

src/unix/linux_like/linux/mod.rs

@@ -1644,6 +1644,8 @@ pub const PR_SET_MDWE: c_int = 65;
 pub const PR_GET_MDWE: c_int = 66;
 pub const PR_MDWE_REFUSE_EXEC_GAIN: c_uint = 1 << 0;
 pub const PR_MDWE_NO_INHERIT: c_uint = 1 << 1;
+pub const PR_SET_MEMORY_MERGE: c_int = 67;
+pub const PR_GET_MEMORY_MERGE: c_int = 68;
 
 pub const GRND_NONBLOCK: c_uint = 0x0001;
 pub const GRND_RANDOM: c_uint = 0x0002;