musl: Move statfs.h and statvfs.h definitions to src/new

[valentynkit]

This follows the ongoing migration into src/new, which reduces
duplication and models musl's source tree.

Arch specific definitions are kept for mips, mips64 and s390x,
mirroring musl's arch/*/bits/statfs.h.

statfs64 is now a type alias for statfs, as it already was on
s390x. musl has no second struct: sys/statfs.h #defines
statfs64 to statfs, and the lfs types fsblkcnt64_t and
fsfilcnt64_t to fsblkcnt_t and fsfilcnt_t. This crate declares
both spellings as c_ulonglong on every musl target, so the duplicated
structs had identical layout.

statvfs64 is now also a type alias for statvfs following the same reasons as before.

Let me know if I should revert the changes for using type alias, and just do the migration without it, but from my findings it looks safe to do.
https://github.com/kraj/musl/blob/1c82569f4cc53f5bb25c820051c2a02463f46cab/include/sys/statfs.h#L21-L26

libc/src/unix/linux_like/linux/musl/mod.rs

Lines 39 to 46 in b99bf9d

	pub type fsblkcnt_t = c_ulonglong;
	pub type fsfilcnt_t = c_ulonglong;
	pub type rlim_t = c_ulonglong;
	// FIXME(1.0,deprecate): lfs binding to be removed
	pub type fsblkcnt64_t = c_ulonglong;
	// FIXME(1.0,deprecate): lfs binding to be removed
	pub type fsfilcnt64_t = c_ulonglong;

Link: https://github.com/kraj/musl/blob/1c82569f4cc53f5bb25c820051c2a02463f46cab/include/sys/statfs.h
Link: https://github.com/kraj/musl/blob/1c82569f4cc53f5bb25c820051c2a02463f46cab/include/sys/statvfs.h
Link: https://github.com/kraj/musl/blob/1c82569f4cc53f5bb25c820051c2a02463f46cab/arch/s390x/bits/statfs.h
Link: https://github.com/kraj/musl/blob/1c82569f4cc53f5bb25c820051c2a02463f46cab/arch/mips/bits/statfs.h
Link: https://github.com/kraj/musl/blob/1c82569f4cc53f5bb25c820051c2a02463f46cab/arch/mips64/bits/statfs.h

[valentynkit]

These changes follow from previous work in this repo, the ongoing migration to src/new suggests contributions like this are useful, but let me know if this isn't wanted

Two things worth attention:

• making statfs64/ statvfs64 type aliases instead of duplicate structs (but some fields use lfs bindings like fsblkcnt64_t)
• musl statvfs has f_type, so it was not added in this PR (glibc side glibc: Add f_type to statvfs and statvfs64 #5434), to keep the move PR separate as it already contains the move of 2 definitions.