libc: upgrade musl from 1.1.24 to 1.2.1

[gburd]

Depends on #1395 (build: modern-toolchain compat). This branch stacks on that PR; until it merges, this PR's diff includes the build-compat commit as well as the musl bump.

Bump the bundled musl from 1.1.24 (2019) to 1.2.1 (2020), the last musl
release with the BSD-style internal layout that OSv's libc shim expects
before musl's 1.2.x source re-architecture. The upgrade picks up roughly
four years of upstream bug fixes in stdio, math, dns, and threading.

This depends on PR 0 (modern-toolchain compat): musl 1.2.1 cannot build
on GCC 14 / Boost 1.87 without it. I verified that the lower_bound /
conditional_t breakage PR 0 fixes is toolchain breakage, not musl's --
bare master fails identically on the same toolchain.

Changes

• Repin the musl_1.1.24 submodule to the public musl v1.2.1 tag
  (73cc775b).
• Add -isystem musl/arch/$(musl_arch) to the musl per-object include
  path: 1.2.x relocated several arch-specific headers there.
• include/api/sys/__socket.h: provide struct msghdr / struct cmsghdr
  (with correct big/little-endian padding) at the common path -- 1.2.x
  moved them out of the arch-specific bits/socket.h.
• libc/stdio/__string_read.c: add OSv's copy of __string_read and move
  it from the musl group to the libc group (it is now referenced from
  OSv-built stdio objects, not the musl tree).
• nftw(): silence a 1.2.1 -Wmaybe-uninitialized and initialise the field
  explicitly so results no longer depend on allocator state (the 1.1.24
  code relied on 1.1.x malloc zeroing).
• Suppress -Wincompatible-pointer-types on tempnam/tmpnam/__map_file
  (musl 1.2.x signatures vs OSv's syscall_to_function.h shim) -- ABI
  unchanged; these are OSv-shim build warnings only.
• Minor aarch64/x64 bits/ header deltas (dirent d_off, socket type)
  to match 1.2.1.

Notes

• No ABI break for OSv apps: the libc layout OSv exposes is unchanged;
  this is a within-1.x musl bump, deliberately stopping at 1.2.1 before
  the 1.2.x internal re-architecture.
• The in-tree libc tests are used as the musl unit-test bar. tst-time's
  TZ cases require a zoneinfo database in the image; on an FHS host this
  comes from /usr/share/zoneinfo and the tests pass. (On a non-FHS host
  the manifest glob can miss tzdata and those cases read UTC -- a
  packaging artifact, not a libc fault.)
• Second in the build/libc modernization series (PR 0 -> this -> virtio
  multi-queue -> ...).