provide loff_t as typedef

[GitMensch]

muslc defines those, but we want a typedef instead

solution copied from libgo
https://gcc.gnu.org/pipermail/gcc-patches/2022-April/593527.html

Note: instead of EmuFS.cc this may go to ScopedFd.h, just drop a note and I'll adjust that.

Also note: clangd warns on this file:

Included header fstream is not used directly (fixes available)
Included header ReplaySession.h is not used directly (fixes available)
Included header core.h is not used directly (fixes available)
Included header kernel_abi.h is not used directly (fixes available)
Included header kernel_metadata.h is not used directly (fixes available)

Fixing the related parts seen in the build log for #4009.