rtld: Reject ELF files with PT_LOAD or PT_TLS segments where filesz > memsz

[jhb]

Summary

All sorts of places in the ELF loading code assume that filesz <=
memsz, so check that explicitly up front. The kernel already performs
this check for the PT_LOAD segments in the main binary and rtld in
imgact_elf.c.

[kib]

I do not object, but I also note that rtld trusts the format of the objects to be correct. There is a lot of missed bounds checks.

[jrtc27]

There's also lib/libc/gen/tls.c for PT_TLS

[jhb]

In D58541#1343668, @jrtc27 wrote:

There's also lib/libc/gen/tls.c for PT_TLS

Oof, thanks for catching that, I'll add another commit for that.